add channel to miri apcorr schema

[braingram]

MIRI MRS AP corr files on CRDS now contain a META.INSTRUMENT.CHANNEL parameter.

https://jwst-crds.stsci.edu/browse/jwst_miri_apcorr_0007.asdf

This PR adds the corresponding keyword to the MIRI MRS AP corr schema to keep things in sync and fix the failing test that compares the crds parameters to the schema.

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 lines are covered by tests :white_check_mark:

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