Blender Git Loki

CMake mini-rewrite: Sync code check with BKE_blender_version_is_alpha

Reproduce the logic we already do in C (BKE_blender_version_is_alpha)
and the CMake file.

Otherwise it can get out of sync if we add/rename the non-alpha release cycles.

No functional change.

Differential Revision: https://developer.blender.org/D13379