Release PHPCSExtra 1.2.1

Release checklist

[x] Verify, and if necessary, update the allowed version ranges for various dependencies in the composer.json - PR #298
[x] Add changelog for the release - PR #300 :pencil2: Remember to add a release link at the bottom!
[x] Update sniff list in README (if applicable) - N/A

[x] Merge this PR
[x] Make sure all CI builds are green.
[x] Tag and create a release (careful, GH defaults to develop!) & copy & paste the changelog to it. :pencil2: Check if anything from the link collection at the bottom of the changelog needs to be copied in!
[x] Make sure all CI builds are green.
[x] Close the milestone
[x] Open a new milestone for the next release
[x] If any open PRs/issues which were milestoned for this release did not make it into the release, update their milestone.
[x] Fast-forward develop to be equal to stable