Align RC names with GitLab
As discussed in https://gitlab.com/gitlab-org/release-tools/issues/131#note_46754253
we will change the version schema for RC:
from v10.3.0-rc.1
to v10.3.0-rc1
without a dot between RC and the number, so that it aligns with GitLab.
My plan is to ~"Pick into 10.3" and releasing a v10.3.0-rc2
in order to test the whole process.