deprecate merge(::Callable ..) in favor of mergewith

[adienes]

after #34296 this method is redundant, and it's also undocumented. according to #43491 (comment), Callable should not be used anymore, so it might make sense to remove from Base where feasible.

[LilithHafner]

From triage: yes, please.

(In general the pattern func(f, collection) where f is called on all the elements of collection before proceeding with func is bad and we wish we had never done that for anything except map)

[adienes]

I see the same test failure on other PRs too so it should be unrelated. merging before the 1.13 branch cuts so I don't have to update this again 🙂