remove unused ignore_version_mismatch

[braingram]

ignore_version_mismatch no longer does anything (see asdf-format/asdf#1812) since it was only used by the legacy asdf extension API removed in asdf 3.0.0.

This PR removes all uses of the setting.

There are no uses of ignore_version_mismatch in jwst: https://github.com/search?q=repo%3Aspacetelescope%2Fjwst%20ignore_version_mismatch&type=code

Checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.21%. Comparing base (83aec51) to head (686125a).
Report is 82 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #313   +/-   ##
=======================================
  Coverage   66.21%   66.21%           
=======================================
  Files         101      101           
  Lines        5402     5402           
=======================================
  Hits         3577     3577           
  Misses       1825     1825           

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

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[nden]

LGTM