JP-3580 Update MRSPRCHN allowed values in keyword schema

[penaguerrero]

Resolves JP-3580

Closes spacetelescope/jwst#8367

This PR addresses adding ALL_MRS to the enum list for keyword MRSPRCHN.

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 64.92%. Comparing base (4d7c3a6) to head (3086334). Report is 13 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #285 +/- ## ========================================== + Coverage 64.84% 64.92% +0.08% ========================================== Files 103 104 +1 Lines 5694 5710 +16 ========================================== + Hits 3692 3707 +15 - Misses 2002 2003 +1 ```

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