Update release notes for Memorystore for Valkey Beta release (#6640)

### BRIEF Change description

Add release note for Memorystore for valkey instance beta release

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Author: xiaoweim

docs/releasenotes/release-1.145.md

@@ -8,6 +8,9 @@
 
 *   Added the `preview` command to the `config-connector` CLI. The `preview` command has been removed from the experimental `kompanion` tool.
 
+## New Beta Resources (Direct Reconciler):
+
+* MemorystoreInstance to manage [Memorystore for Valkey Instances](https://cloud.google.com//memorystore/docs/valkey/reference/rest/v1/projects.locations.instances)
 
 ## Bug Fixes: