[8.9] Add deprecated note for balanced allocator (#98610) (#98615)

Co-authored-by: James Rodewig <[email protected]>
Co-authored-by: Roberto Seldner <[email protected]>

docs/reference/modules/cluster/shards_allocation.asciidoc

@@ -115,6 +115,7 @@ runs a background task which computes the desired balance of shards in the
 cluster. Once this background task completes, {es} moves shards to their
 desired locations.
 
+deprecated:[8.8,The `balanced` allocator type is deprecated and no longer recommended]
 May also be set to `balanced` to select the legacy _balanced allocator_. This
 allocator was the default allocator in versions of {es} before 8.6.0. It runs
 in the foreground, preventing the master from doing other work in parallel. It