Mathtext's \stackrel{a}{b} behaves subtly differently from latex's
\stackrel{a}{b}: mathtext makesa andb equal-sized whereas latex
makesa smaller thanb (a is an "annotation" overb). 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 isFlake 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