Add deprecation warnings beyond the README

[dmsimard]

Hi,

7fa688d added a deprecation notice to the README but users won't know about it unless they browse the git repository.

We are nearing the release of Ansible 5 (of which this collection will still be a part of, until Ansible 6) and it would be nice if we could add a deprecation notice to the plugins in this collection and then get a release out in time so that users can get the deprecation warning.

Edit: We should also add a deprecated_features changelog fragment that would mention the deprecation of the collection so that it shows up in release changelogs and porting guides.

[willtome]

@dmsimard can you advise on how to get CI passing for these deprecation notices? I'm getting lost in all the different places it needs to be included.