Fixup: fix codec backward compatibility tests

We use a custom JSON serializer for remote signature which can only be full (64 bytes) signatures.

Author: sstone

eclair-core/src/main/scala/fr/acinq/eclair/json/JsonSerializers.scala

@@ -332,6 +332,11 @@ object ColorSerializer extends MinimalSerializer({
   case c: Color => JString(c.toString)
 })
 
+// @formatter:off
+private case class CommitTxAndRemoteSigJson(commitTx: CommitTx, remoteSig: ByteVector64)
+object CommitTxAndRemoteSigSerializer extends ConvertClassSerializer[CommitTxAndRemoteSig](i => CommitTxAndRemoteSigJson(i.commitTx, i.remoteSig.asInstanceOf[RemoteSignature.FullSignature].sig))
+// @formatter:on
+
 // @formatter:off
 private sealed trait HopJson
 private case class ChannelHopJson(nodeId: PublicKey, nextNodeId: PublicKey, source: HopRelayParams) extends HopJson
@@ -711,6 +716,7 @@ object JsonSerializers {
     OpenChannelResponseSerializer +
     CommandResponseSerializer +
     InputInfoSerializer +
+    CommitTxAndRemoteSigSerializer +
     ColorSerializer +
     ThrowableSerializer +
     FailureMessageSerializer +

eclair-core/src/test/scala/fr/acinq/eclair/json/JsonSerializersSpec.scala

@@ -204,7 +204,7 @@ class JsonSerializersSpec extends TestKitBaseClass with AnyFunSuiteLike with Mat
         |         "localCommit": {
         |           "index": 0,
         |           "spec": { "htlcs": [], "commitTxFeerate": 2500, "toLocal": 100000000, "toRemote": 50000000 },
-        |           "commitTxAndRemoteSig": { "commitTx": { "txid": "4ebd325a4b394cff8c57e8317ccf5a8d0e2bdf1b8526f8aad6c8e43d8240621a", "tx": "02000000000000000000" }, "remoteSig": { "sig": "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" } },
+        |           "commitTxAndRemoteSig": { "commitTx": { "txid": "4ebd325a4b394cff8c57e8317ccf5a8d0e2bdf1b8526f8aad6c8e43d8240621a", "tx": "02000000000000000000" },"remoteSig": "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" },
         |           "htlcTxsAndRemoteSigs": []
         |         },
         |         "remoteCommit": {