releases: add one br entry to v8.1.2 (#20268)

Author: BornChanger

releases/release-8.1.2.md

@@ -39,6 +39,7 @@ TiDB 版本：8.1.2
     + Backup & Restore (BR)
 
         - 减少备份过程中无效日志的打印 [#55902](https://github.com/pingcap/tidb/issues/55902) @[Leavrth](https://github.com/Leavrth)
+        - 在进行全量备份时，默认不计算表级别的 checksum (`--checksum=false`) 以提升备份性能 [#56373](https://github.com/pingcap/tidb/issues/56373) @[Tristan1900](https://github.com/Tristan1900)
 
     + TiCDC