Updating the Sponza and ReferenceMaterials CMake targets. (#8307)
The AtomContent_Sponza and AtomContent_ReferenceMaterials CMake targets
have been updated to match the names of the Gems.
This allows using the gems in the enabled_gems.cmake list to activate
the gem variants based off of the name Sponza and ReferenceMaterials.
Signed-off-by: lumberyard-employee-dm <[email protected]>