FindRuby: Fix vendor directory discovery

The `ruby -r vendor-specific` module no longer exists.
Use `ruby -r rbconfig` instead.

This patch has been carried by openSUSE's cmake package:

* https://build.opensuse.org/projects/openSUSE:Factory/packages/cmake/files/cmake-fix-ruby-test.patch?rev=274
* https://build.opensuse.org/package/rdiff/openSUSE:Factory/cmake?rev=106
* https://build.opensuse.org/package/rdiff/openSUSE:Factory/cmake?rev=57
* https://build.opensuse.org/package/rdiff/openSUSE:Factory/cmake?rev=56

Fixes: #27888