CMakeVersion: Use '-rc0' version suffix on release branches prior to rc1
Revert the change from !3603 (merged) and instead define a 0
value in CMake_VERSION_RC
to mean -rc0
. This distinguishes release branch versions prior to the first release candidate from the first release candidate itself. It also makes room for a dedicated CMake $major.$minor.0-rc1
release commit for -rc1
as we have for later release candidates and final releases.