VCS: deprecation dates at application level (#11147)
* VCS: deprecation dates at application level
Follow the dates published in our blog:
https://about.readthedocs.com/blog/2024/02/drop-support-for-subversion-mercurial-bazaar/
Closes #8840
* Minor updates after doing more QA locally
* Update readthedocs/notifications/messages.py
Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>
* Typo
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Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>
