PR Summary

This also allows us to sort all extensions in alphabetical order, in line with the other backends, rather than having to put the default option first.

This change required raising the minimum supported version of Tk to 8.5, as discussed in #24450. I also removed a try statement around iconphoto, which should only have failed on Tk < 8.5 - it didn't feel big enough to deserve its own PR, but happy to separate it out if anyone disagrees.

Closes #24450.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] New plotting related features are documented with examples.

Release Notes

• [N/A] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst