[libc++] Encode the version in the ReleaseNotes file

This way, we don't need to remove the contents of the ReleaseNotes file
after the branch. This should make it much easier and natural to cherry-pick
changes onto the release branch. Typically, we need two patches for those.
First, we need the code changes against `main`, and then we need a patch
that updates the release notes on the just-created branch.

By versioning the release notes, it becomes easy to author a change
against `main` that targets a just-branched LLVM release by simply
adding it to the release notes for the right version. This has been
a pain point in previous releases.

Differential Revision: https://reviews.llvm.org/D155024