[Cherry-pick] [RLlib, RLlib contrib] add soft deprecation notices to maml and a3c

[avnishn]

…#35345)

Cherry picking some deprecation notices for the 2.5 release

Why are these changes needed?

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[zhe-thoughts]

@avnishn could you link the original PR that you are cherry picking from? Thanks

[avnishn]

sure @zhe-thoughts
#35345

[zhe-thoughts]

Thanks @avnishn . Approved. I'm updating the PR title to match the master PR