deprecate get_fileext

[braingram]

As noted in #16 DataModel.get_fileext is unused.

This PR deprecates it for removal in an upcoming release.

Regression tests all pass: https://github.com/spacetelescope/RegressionTests/actions/runs/11863222622 and the logs do not contain the warning added in this PR (confirming no use in jwst).

Tasks

• [ ] update or add relevant tests
• [ ] update relevant docstrings and / or docs/ page
• [ ] Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • [ ] write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • [ ] run jwst regression tests with this branch installed ("git+https://github.com/<fork>/stdatamodels@<branch>")

news fragment change types...

- ``changes/.feature.rst``: new feature - ``changes/.bugfix.rst``: fixes an issue - ``changes/.doc.rst``: documentation change - ``changes/.removal.rst``: deprecation or removal of public API - ``changes/.misc.rst``: infrastructure or miscellaneous change

[codecov[bot]]

Codecov Report

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

Project coverage is 67.55%. Comparing base (1e16207) to head (a70d4ea). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #363 +/- ## ========================================== + Coverage 67.52% 67.55% +0.02% ========================================== Files 114 114 Lines 5916 5917 +1 ========================================== + Hits 3995 3997 +2 + Misses 1921 1920 -1 ```

:umbrella: View full report in Codecov by Sentry.
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