PR https://github.com/ruby/ruby/pull/3418 added a category to
`rb_warn_deprecated_to_remove` but not to `rb_warn_deprecated`. This
adds the same code to `rb_warn_deprecated` so that those warnings also
get a category.

This change also adds tests for `rb_warn_deprecated` and updates the
tests for `rb_warn_deprecated_to_remove` to have clearer names.

I've fixed the call to `rb_method_entry` as we need to be using the
instance method, not singleton.

Feature: https://bugs.ruby-lang.org/issues/17122