[clang] [clang-tools-extra] [flang] [lld] [llvm] [LLVM][utils] Add script which clears release notes (PR #153593)

[Tobias Hieta via cfe-commits]

tru wrote:

Maybe out of scope for this PR - but I wonder if we should just copy the release notes to ReleaseNotes-LLVM21.rst instead of just nuking them from main. That would make it easier to see what recently happened and not much of a difference for the script.

What do you guys think @tstellar @AaronBallman @nikic ?

https://github.com/llvm/llvm-project/pull/153593