deprecate `merge(::Callable ..)` in favor of `mergewith` (#59775)
after https://github.com/JuliaLang/julia/pull/34296 this method is
redundant, and it's also undocumented. according to
https://github.com/JuliaLang/julia/issues/43491#issuecomment-1004364287,
`Callable` should not be used anymore, so it might make sense to remove
from `Base` where feasible.
