revert: "release: v12.0.0"

[maribethb]

Reverts #2531

These changes were released as new patch versions for each plugin instead of new major versions. Plan is to

1. Revert this merge, check that I don't need to do anything related to fix: Don't pin continuous-toolbox to Blockly v12.0.0-beta.0. #2532. This should put all plugins back on v11.
2. Publish all plugins again (as new patch version)
3. Submit chore: fix lerna release config #2533 so that next publish will be a major version as intended
4. Re-merge rc/v12.0.0 branch (don't want to just unrevert this PR because I need to make sure there's a breaking change in there for lerna)
5. Check fix: Don't pin continuous-toolbox to Blockly v12.0.0-beta.0. #2532 again
6. Publish all plugins again (now should be updated to v12 and new major version)