Release version 2.2.1

[jeffpaul]

This issue is for tracking changes for the 2.2.1 release. Target release date: TBD.

Pre-release steps

• [ ] Complete review of PR #86, resolve issues/merge

Release steps

• [x] Branch: Starting from develop, cut a release branch named release/2.2.1 for your changes.
• [x] Version bump: Bump the version number in src/bootstrap.php if it does not already reflect the version being released.
• [x] Changelog: Add/update the changelog in CHANGELOG.md.
• [x] Props: Update CREDITS.md file with any new contributors, confirm maintainers are accurate
• [x] Readme updates: Make any other readme changes as necessary in README.md.
• [ ] Merge: Make a non-fast-forward merge from your release branch to develop (or merge the pull request), then do the same for develop into master (git checkout master && git merge --no-ff develop). master contains the stable development version.
• [ ] Push: Push your master branch to GitHub (e.g. git push origin master).
• [ ] Release: Create a new release, naming the tag and the release as 2.2.1, and targeting the master branch. Paste the changelog from CHANGELOG.md into the body of the release and include a link to the closed issues on the milestone. The release should now appear under releases.
• [ ] Wait for build: Head to the Actions tab in the repo and wait for it to finish if it hasn't already. If it doesn't succeed, figure out why and start over. The new images should now appear under tags as 2, 2.2.1, and latest.
• [ ] Close milestone: Edit the milestone with release date (in the Due date (optional) field) and link to GitHub release (in the Description field), then close the milestone.
• [ ] Punt incomplete items: If any open issues or PRs which were milestoned for 2.2.1 do not make it into the release, update their milestone to 2.3.0 or Future Release.

Post-release steps

• [ ] Update WP Acceptance's Docker image with this new WP Snapshots version (2.2.1)