Fixed #23581 -- Prevented extraneous DROP DEFAULT statements.

Thanks john_scott for the report and Markus Holtermann for review.
Added test for an intermediate swappable model change in migration state.

refs #22563
Fixed #12118 -- Added shared cache support to SQLite in-memory testing.
Removed unused import.
Fixed #23450 -- Fixed transient failure of requests...test_far_expiration.
Revert "Updated some docs for the delayed deprecation of legacy table creation; refs #22340."

This reverts commit a2e3c9694823d6d451d6dd234e9d416600aacc68.

The deprecation was moved back to 1.9 in
61da5f3f02f34810aaa6fcddac3808318a5b95c4.
Removed db.backends methods which only call super().
Added ignore_warnings decorator

And removed Ignore*DeprecationWarningsMixin, now obsolete.
Thanks Berker Peksag and Tim Graham for the review.
Applied ignore_warnings to Django tests