Prep for v1.21.1

[odow]

Pre-release

• [x] 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.
• [x] 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.
• [x] Check compat of DimensionalData in Project.toml
• [x] Check compat of MacroTools in Project.toml
• [x] Update docs/src/changelog.md
• [x] https://github.com/jump-dev/JuMP.jl/actions/runs/8637630824
• [x] Change the version number in Project.toml
• [x] The commit messages in this PR do not contain [ci skip]

The release

• [x] 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

• [x] 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