Release Thunder 8.2.1

Problem/Motivation

Release Thunder 8.2.1

Remaining tasks

1. Create release branch from 8.2.x
2. Write release notes
3. Bump version string in thunder.info.yml
4. If necessary, update Drupal Core version in composer.json
5. Double check, if fixed versions of patched modules should be updated.
6. Merge release branch
7. Tag the merge commit as 8.2.1
8. Create release on drupal.org
9. Announce release