releases: add one br entry to v8.1.2 (#20861)

Author: BornChanger

releases/release-8.1.2.md

@@ -39,6 +39,7 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.1/quick-start-with-
     + Backup & Restore (BR)
 
         - Reduce unnecessary log printing during backup [#55902](https://github.com/pingcap/tidb/issues/55902) @[Leavrth](https://github.com/Leavrth)
+        - Disable the table-level checksum calculation (`--checksum=false`) during full backups by default to improve backup performance [#56373](https://github.com/pingcap/tidb/issues/56373) @[Tristan1900](https://github.com/Tristan1900)
 
     + TiCDC