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/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change