docs: release notes for new RPC getislocks and for RPC getbestchainlock changes

Author: knst

doc/release-notes-6455.md

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+## New RPCs
+
+- **`getislocks`**
+    - Retrieves the InstantSend lock data for the given transaction IDs (txids).
+    Returns the lock information in both human-friendly JSON format and binary hex-encoded zmq-compatible format.
+
+## Updated RPCs
+
+- **`getbestchainlock` Changes**
+    - A new hex field has been added to the getbestchainlock RPC, which returns the ChainLock information in zmq-compatible, hex-encoded binary format.
+