When partitioning version constants into released and unreleased
versions, today we have a bug in finding the last unreleased
version. Namely, consider the following version constants on the 6.x
branch: ..., 5.6.3, 5.6.4, 6.0.0-alpha1, ..., 6.0.0-rc1, 6.0.0-rc2,
6.0.0, 6.1.0. In this case, our convention dictates that: 5.6.4, 6.0.0,
and 6.1.0 are unreleased. Today we correctly detect that 6.0.0 and 6.1.0
are unreleased, and then we say the previous patch version is unreleased
too. The problem is the logic to remove that previous patch version is
broken, it does not skip alphas/betas/RCs which have been released. This
commit fixes this by skipping backwards over pre-release versions when
finding the previous patch version to remove.
Relates #27206
d6d830ff0b