Deprecate Pathname#/ with non-string/non-pathname arguments

Ruby 2.2 will define Pathname#/ as a simple alias of Pathname#+.
In practice, this means that it will raise a TypeError unless the
argument responds to to_path or to_str.

Currently we blindly convert the argument to a string using to_s, so
deprecate this in the interest of matching the upstream behavior. In the
future we can replace this with

  alias_method :/, :+ unless method_defined?(:/)

Closes #30079.
Deprecate Pathname#cp and Pathname#chmod_R

As far as I can tell these methods have only ever been used in the test
suite.

Since Formula includes FileUtils, it is generally simpler (and in the
case of cp, more readable) to use the FileUtils methods directly.

Closes #30081.