JP-3581: add badpix_selfcal step to core schema

[emolter]

Resolves JP-3581

Closes #8369

This PR adds a new header keyword to the cal_step in the core schema to reflect the addition of the new badpix_selfcal step in JWST.

Checklist

• [x] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [x] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.22%. Comparing base (4d7c3a6) to head (a8e8665). Report is 61 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #305 +/- ## ========================================== + Coverage 64.84% 66.22% +1.38% ========================================== Files 103 101 -2 Lines 5694 5403 -291 ========================================== - Hits 3692 3578 -114 + Misses 2002 1825 -177 ```

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[braingram]

Changes LGTM.

I suggest we leave this open until the keyword changes make this "official" (in case some unforeseen name change occurs). See: https://github.com/spacetelescope/jwst/pull/8500#discussion_r1639940884