FindRuby: Rename variables to match case of module name

Use recommended case for variable names, i.e. matching name of the
module as passed to `find_package`.

For backwards compatibility, the upper case versions of both input and
output variables are used and defined when appropriate.  Skip this for
the _FOUND variable because FPHSA already does it.

This follows the approach from commit a7b09e7f (FindProtobuf: Rename
variables to match case of module name, 2016-03-01, v3.6.0-rc1~273^2).

Issue: #20370