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 86.07%. Comparing base (33154e4) to head (006ba27).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   86.94%   86.07%   -0.87%     
==========================================
  Files          42       41       -1     
  Lines        2628     2586      -42     
==========================================
- Hits         2285     2226      -59     
- Misses        343      360      +17     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.