Previously, `parent` was added as one of the RESTRICTED_CLASS_METHODS as
part of a commit (94b7328b) in 2014 that stopped Rails `enum`s from being
able to redefine important class methods.

At the time, Rails monkey-patched `Module` with a `parent` class method
that returned a module's containing module if it was nested.

However, in October 2020 (167b4153) in Rails 6.1, this method was
deprecated in favour of a renamed method `module_parent`. As such, the
`parent` method doesn't need to be a restricted class method any more.