Update deprecation sections to reflect new policy

[WardBrian]

Submission Checklist

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

Summary

Update deprecation sections to reflect stan-dev/design-docs#43

Related to stan-dev/stanc3#1044

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 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bob-carpenter]

thanks---just a few minor comments.

[bob-carpenter]

Thanks.

[bob-carpenter]

It's still marked as draft, but if your intention is to merge, you should be able to do that now.

[WardBrian]

@bob-carpenter I'm going to see if any further documentation needs come up in the next couple stanc PRs related to this before merging, but if nothing changes I'll merge after those.