Mathtext's \stackrel{a}{b} behaves subtly differently from latex's
\stackrel{a}{b}: mathtext makes a and b equal-sized whereas latex
makes a smaller than b (a is an "annotation" over b). Given
that we can already stack expressions using the (admittedly less
practical, but standard) \genfrac, just deprecate \stackrel instead of
maintaining our own latex-like dialect.

Also undeprecate passing obj_type to the cbook.deprecated decorator(!),
given that this PR has a good reason to use it... and while we're at it,
reorder the kwargs docs for cbook.deprecated and cbook.warn_deprecated
to match the order in the signature.

Again related to #12108 / #12128 (:mathmpl: still exists because mathtext is subtly different from latex).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way