DEP: deprecate and remove remaining usages of slur-adjacent "trapz"

[h-vetinari]

I am not finding any other appearances of these terms except for trapz, which is still allowed as an alias for the trapezoidal distribution and as a method option of quad_vec. (Those uses don't appear to have been deprecated.)

Originally posted by @mdhaber in #20278 (review)

[j-bowhay]

I will address this before the next release if nobody gets there first

[rgommers]

thanks @h-vetinari

which is still allowed as an alias for the trapezoidal distribution

IIRC we kept it as an undocumented alias when introducing trapezoidal to avoid users having to write if-else constructs depending on SciPy version. That is quite a few releases ago, so +1 for the proposal deprecations now.