FutureWarning for sdba migration to xsdba

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR partially satisfies Split xlim.sdba into xsdba #2074
  • This PR partially satisfies Xclim Development Goals 2025 #2047
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Adds a FutureWarning emitted on import of xclim.sdba
• Adds Warning messages to the API docs page and autogenerated documentation.
• Adds a workflow that posts a warning message on Pull Requests that modify the xclim.sdba module. Without approval, merging is explicitly blocked.

Does this PR introduce a breaking change?

Yes. xclim.sdba is now officially in the process of being deprecated.

Other information:

https://github.com/Ouranosinc/xsdba