gz-rendering9+: build without -DSKIP_optix

[scpeters]

This was added in #456, but now it seems to be causing a cmake warning for an unused manually specified variable:

• https://build.osrfoundation.org/job/gz_rendering-ci-main-noble-amd64/24/cmake/

CMake Warning:
  Manually-specified variables were not used by the project:

    SKIP_optix

testing with this branch:

• 1 test failure expected, but no cmake warnings https://build.osrfoundation.org/job/gz_rendering-ci-main-noble-amd64/25/cmake/

[scpeters]

never mind, this introduces a cmake warning for ign-rendering3

• https://build.osrfoundation.org/job/gz_rendering-ci-ign-rendering3-focal-amd64/65/cmake/

[scpeters]

oh, the cmake warning was introduced by gazebosim/gz-rendering#1032; I'll focus my attention there

[scpeters]

actually, let me add some version logic to this

[scpeters]

cmake warnings look fixed with the version logic added in

• citadel: https://build.osrfoundation.org/job/gz_rendering-ci-ign-rendering3-focal-amd64/66/
• ionic (one expected test failure, no cmake warnings): https://build.osrfoundation.org/job/gz_rendering-ci-main-noble-amd64/26/cmake/