[llvm] Change release branch creation process to bump version to N.1.0. (PR #75743)

[via llvm-commits]

h-vetinari wrote:

> I don't have any objections to this

Out of curiosity, in the RFC you [said](https://discourse.llvm.org/t/rfc-name-the-first-release-from-a-branch-n-1-0-instead-of-n-0-0/75384/18): 
> I think if we implement this proposal, we should also add the minor version to the SOVERSION. N.0.0 (trunk) would have a different ABI than N.1.0 (release branch).

Shouldn't this be done (or at the very least documented) with this PR? Otherwise it risks slipping through the cracks IMO.

https://github.com/llvm/llvm-project/pull/75743