Prepare release notes for "master"

This merge request attempts to achieve three objectives:

• Forward-port tweaks and changes applied to release notes and CHANGES in !3495 (merged)/!3496 (merged)/!3497 (merged) to master.

• Restore all tweaks applied to the release notes on the master branch since !1761 (merged) was originally branched off from it.

• Once !3516 (closed) gets merged, this MR should make backporting release notes to v9_16 easier as the source file containing release notes for the next release will now always be called doc/arm/notes-current.rst.txt and no version numbers will be embedded in that file. The idea is to move release notes for a given release to a version-specific file right before tagging, cleaning doc/arm/notes-current.rst.txt in the process.