Change release branch creation process to bump version to N.1.0. (#75743)

This will help distinguish release branch builds from development branch
builds, and is similar to GCC's version numbering policy.

Thus, the branch `releases/18.x` will start out numbered 18.1.0, instead
of 18.0.0.

Unchanged are other versioning policies:
- mainline will be numbered 18.0.0, 19.0.0, ...
- typical release branch releases will increment micro version, e.g.
18.1.1, 18.1.2, ....
- If an ABI break is required on the release branch, the minor version
will be incremented, e.g. to 18.2.0.

See the Discourse RFC:

https://discourse.llvm.org/t/rfc-name-the-first-release-from-a-branch-n-1-0-instead-of-n-0-0/75384