Set cluster size hint

[ChunyiLyu]

This closes #

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

• set cluster_formation.target_cluster_size_hint. The property is currently used in importing definitions: Import queue and binding definitions when a certain number of nodes join rabbitmq-server#3981
• update github action to test again 3.9.9 and README

Additional Context

• There needs to be release notes added when next operator version is cut as this is explicitly changing the minimal support RMQ version (RMQ without this configuration will fail to deploy).

Local Testing