[3.6.x] Modernize Release Flow

[jackkoenig]

I did this quickly and a little bit slash-and-burn-y, but @jerryz123 requested 3.6.1 to help Chipyard. The legacy release flow is too much work to maintain so I went ahead and applied the same modernizations to 3.6.x that we did in 5.x. Once merged, a 3.6.0+50- will be published and Jerry can test with it, if it works, we can publish 3.6.1 from this repo rather than the legacy flow.

Note this must be merged with a merge commit as it is intentionally merging in the commit history from 3.6-release which contains the actual commit tagged v3.6.0 which is needed for ci-release to work properly.

• Pin firrtl version to 1.6.0
• Pin chiseltest version used in testing to 0.6.2
• Switch to unipublish
• Remove 3.6.0 as MiMa previous version as unipublish won't work with it
  Will restore checks with 3.6.1 release, this is mitigated by the fact
  that every backport to this point has been checked.
• Update Github Actions versions
• Remove CI for documentation and website (handled by main branch)

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.