Prep for v1.21.0

[odow]

TODO

• DNM: Add unsafe_(g|s)et_backend_attribute #3685
• Add _CONSTRAINT_LIMIT_FOR_PRINTING to control printing of many constraints #3686
• Add batched modification methods #3716

Pre-release

• Check that the pinned packages in docs/Project.toml are updated. We pin
  the versions so that changes in the solvers (changes in printing, small
  numeric changes) do not break the printing of the JuMP docs in arbitrary
  commits.
• Check that the rev fields in docs/packages.toml are updated. We pin
  the versions of solvers and extensions to ensure that changes to their
  READMEs do not break the JuMP docs in arbitrary commits, and to ensure
  that the versions are compatible with the latest JuMP and
  MathOptInterface releases.
• Check compat of DimensionalData in Project.toml
• Check compat of MacroTools in Project.toml
• Update docs/src/changelog.md
• https://github.com/jump-dev/JuMP.jl/actions/runs/8457321785
• Change the version number in Project.toml
• The commit messages in this PR do not contain [ci skip]

The release

• After merging this pull request, comment [at]JuliaRegistrator register in
  the GitHub commit. This should automatically publish a new version to the
  Julia registry, as well as create a tag, and rebuild the documentation
  for this tag.

  These steps can take quite a bit of time (1 hour or more), so don't be
  surprised if the new documentation takes a while to appear. In addition,
  the links in the README will be broken until JuliaHub fetches the new
  version on their servers.
  

Post-release

• Once the tag is created, update the relevant release- branch. The latest
  release branch at the time of writing is release-1.0 (we haven't
  back-ported any patches that needed to create a release-1.Y branch). To
  to update the release branch with the v1.10.0 tag, do:
  git checkout release-1.0 git pull git merge v1.10.0 git push

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.42%. Comparing base (c3cefe0) to head (c9de5c3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3718      +/-   ##
==========================================
+ Coverage   98.37%   98.42%   +0.04%     
==========================================
  Files          43       43              
  Lines        5737     5828      +91     
==========================================
+ Hits         5644     5736      +92     
+ Misses         93       92       -1     

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[odow]

I don't plan to wait for #3685

[odow]

Merging because we had talked about this on the developer call.