CompatHelper: bump compat for JET in [extras] to 0.12, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the JET package from 0.9, 0.10, 0.11 to 0.9, 0.10, 0.11, 0.12.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.72%. Comparing base (37bab0a) to head (59340ea).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #827      +/-   ##
==========================================
- Coverage   90.74%   87.72%   -3.02%     
==========================================
  Files          11       10       -1     
  Lines        1070      986      -84     
==========================================
- Hits          971      865     -106     
- Misses         99      121      +22     

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