[ci] Rename other stale branch workflow

Makes it easier to tell what is what

Author: poteto

.github/workflows/shared_cleanup_branch_caches.yml renamed to .github/workflows/shared_cleanup_merged_branch_caches.yml

@@ -1,6 +1,6 @@
 # https://github.com/actions/cache/blob/main/tips-and-workarounds.md#force-deletion-of-caches-overriding-default-cache-eviction-policy
 
-name: (Shared) Cleanup Branch Caches
+name: (Shared) Cleanup Merged Branch Caches
 on:
   pull_request:
     types: