Update Bitcoin Core to v31.1 (#3327)

diff --git a/README.md b/README.md
index fbe2255..495867f 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,7 @@ This means that instead of re-implementing them, Eclair benefits from the verifi
 
 * Eclair needs a _synchronized_, _segwit-ready_, **_zeromq-enabled_**, _wallet-enabled_, _non-pruning_, _tx-indexing_ [Bitcoin Core](https://github.com/bitcoin/bitcoin) node.
 * You must configure your Bitcoin node to use `bech32` or `bech32m` (segwit) addresses. If your wallet has "non-segwit UTXOs" (outputs that are neither `p2sh-segwit`, `bech32` or `bech32m`), you must send them to a `bech32` or `bech32m` address before running Eclair.
-* Eclair requires Bitcoin Core 30 or higher. If you are upgrading an existing wallet, you may need to create a new address and send all your funds to that address.
+* Eclair requires Bitcoin Core 31 or higher. If you are upgrading an existing wallet, you may need to create a new address and send all your funds to that address.
 
 Run bitcoind with the following minimal `bitcoin.conf`:
 
diff --git a/docs/release-notes/eclair-vnext.md b/docs/release-notes/eclair-vnext.md
index a5a9998..bc1f434 100644
--- a/docs/release-notes/eclair-vnext.md
+++ b/docs/release-notes/eclair-vnext.md
@@ -4,7 +4,10 @@
 
 ## Major changes
 
-<insert changes>
+### Update minimal version of Bitcoin Core
+
+With this release, eclair requires using Bitcoin Core 31.x.
+Newer versions of Bitcoin Core may be used, but have not been extensively tested.
 
 ### Configuration changes
 
diff --git a/eclair-core/pom.xml b/eclair-core/pom.xml
index 849d6cb..8108813 100644
--- a/eclair-core/pom.xml
+++ b/eclair-core/pom.xml
@@ -87,8 +87,8 @@
                 <activeByDefault>true</activeByDefault>
             </activation>
             <properties>
-                <bitcoind.url>https://bitcoincore.org/bin/bitcoin-core-30.2/bitcoin-30.2-x86_64-linux-gnu.tar.gz</bitcoind.url>
-                <bitcoind.sha256>6aa7bb4feb699c4c6262dd23e4004191f6df7f373b5d5978b5bcdd4bb72f75d8</bitcoind.sha256>
+                <bitcoind.url>https://bitcoincore.org/bin/bitcoin-core-31.1/bitcoin-31.1-x86_64-linux-gnu.tar.gz</bitcoind.url>
+                <bitcoind.sha256>b80d9c3e04da78fb6f0569685673418cf686fadba9042d926d13fb87ff503f9e</bitcoind.sha256>
             </properties>
         </profile>
         <profile>
@@ -99,8 +99,8 @@
                 </os>
             </activation>
             <properties>
-                <bitcoind.url>https://bitcoincore.org/bin/bitcoin-core-30.2/bitcoin-30.2-x86_64-apple-darwin.tar.gz</bitcoind.url>
-                <bitcoind.sha256>99d5cee9b9c37be506396c30837a4b98e320bfea71c474d6120a7e8eb6075c7b</bitcoind.sha256>
+                <bitcoind.url>https://bitcoincore.org/bin/bitcoin-core-31.1/bitcoin-31.1-x86_64-apple-darwin.tar.gz</bitcoind.url>
+                <bitcoind.sha256>bc506958d0f387c1ea770bdc7c7192a505fa645ff62cabcc7761fa7eb89e867e</bitcoind.sha256>
             </properties>
         </profile>
         <profile>
@@ -111,8 +111,8 @@
                 </os>
             </activation>
             <properties>
-                <bitcoind.url>https://bitcoincore.org/bin/bitcoin-core-30.2/bitcoin-30.2-win64.zip</bitcoind.url>
-                <bitcoind.sha256>0d7e1f16f8823aa26d29b44855ff6dbac11c03d75631a6c1d2ea5fab3a84fdf8</bitcoind.sha256>
+                <bitcoind.url>https://bitcoincore.org/bin/bitcoin-core-31.1/bitcoin-31.1-win64.zip</bitcoind.url>
+                <bitcoind.sha256>c99ef173471c58e6766d9eebd12e6c35349082eeed3939bc99eed58ef57db587</bitcoind.sha256>
             </properties>
         </profile>
     </profiles>
diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/Setup.scala b/eclair-core/src/main/scala/fr/acinq/eclair/Setup.scala
index b5440fc..c68f2ff 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/Setup.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/Setup.scala
@@ -195,7 +195,7 @@ class Setup(val datadir: File,
       await(getBitcoinStatus(bitcoinClient), 30 seconds, "bitcoind did not respond after 30 seconds")
     }
     logger.info(s"bitcoind version=${bitcoinStatus.version}")
-    assert(bitcoinStatus.version >= 300000, "Eclair requires Bitcoin Core 30 or higher")
+    assert(bitcoinStatus.version >= 310000, "Eclair requires Bitcoin Core 31 or higher")
     bitcoinStatus.unspentAddresses.foreach { address =>
       val isSegwit = addressToPublicKeyScript(bitcoinStatus.chainHash, address).map(script => Script.isNativeWitnessScript(script)).getOrElse(false)
       assert(isSegwit, s"Your wallet contains non-segwit UTXOs (e.g. address=$address). You must send those UTXOs to a segwit address to use Eclair (check out our README for more details).")
diff --git a/eclair-core/src/test/scala/fr/acinq/eclair/blockchain/bitcoind/BitcoindService.scala b/eclair-core/src/test/scala/fr/acinq/eclair/blockchain/bitcoind/BitcoindService.scala
index 6ca2a7a..d81eb6f 100644
--- a/eclair-core/src/test/scala/fr/acinq/eclair/blockchain/bitcoind/BitcoindService.scala
+++ b/eclair-core/src/test/scala/fr/acinq/eclair/blockchain/bitcoind/BitcoindService.scala
@@ -62,7 +62,7 @@ trait BitcoindService extends Logging {
 
   val PATH_BITCOIND = sys.env.get("BITCOIND_DIR") match {
     case Some(customBitcoinDir) => new File(customBitcoinDir, "bitcoind")
-    case None => new File(TestUtils.BUILD_DIRECTORY, "bitcoin-30.2/bin/bitcoind")
+    case None => new File(TestUtils.BUILD_DIRECTORY, "bitcoin-31.1/bin/bitcoind")
   }
   logger.info(s"using bitcoind: $PATH_BITCOIND")
   val PATH_BITCOIND_DATADIR = new File(INTEGRATION_TMP_DIR, "datadir-bitcoin")