This PR bumps the version number and updates the changelog after the v2.21.5 release.

Please do the following:

• Remove and re-add the "Update dependencies" label to the PR to trigger just this workflow.
• Wait for the "Update dependencies" workflow to push a commit updating the dependencies.
• Mark the PR as ready for review to trigger the full set of PR checks.
• Approve and merge the PR. When merging the PR, make sure "Create a merge commit" is
  selected rather than "Squash and merge" or "Rebase and merge".