Deprecate or remove extra_fits

[braingram]

As noted in https://jira.stsci.edu/browse/JP-2056 the documentation for extra_fits incorrectly refers to it as _extra_fits:
https://stdatamodels.readthedocs.io/en/latest/jwst/datamodels/models.html#extra-fits-keywords

There are other references to extra_fits:
https://stdatamodels.readthedocs.io/en/latest/search.html?q=extra_fits&check_keywords=yes&area=default

We should consider deprecating extra_fits as part of the "public" API (it appears to only have a single use in jwst):
https://github.com/spacetelescope/jwst/blob/33fd9fa52debf69ccb53aab32ab1073425bc4418/jwst/model_blender/tests/test_blend.py#L127

We should continue to "roundtrip" non-schema fits data but allowing users to arbitrarily update these data while in memory seems dangerous. Any updates can instead use the existing astropy.io.fits API.

Since extra_fits appears in the documentation I suggest we:

• deprecate it
• update the documentation
• remove it in a later major version (I'm milestoneing this issue for 2.0.0)