[Stan 2.33] Update documentation on expiring deprecations

[WardBrian]

Submission Checklist

• Builds locally
• New functions marked with `r since("VERSION")`
• Declare copyright holder and open-source license: see below

Summary

Documentation changes to go with stan-dev/stanc3#1287. Quite a few deprecations are expiring this version and have been moved to the "Removed Features" or "Removed Functions" pages as a result.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)

[WardBrian]

This has been updated following #629, but should not be merged until during the 2.33 development window

[mitzimorris]

looks great! many thanks.