[clang-tidy] Add cppcoreguidelines-prefer-member-initializer to ReleaseNotes
Following a discussion about the current state of this check on the 12.X branch, it was decided to purge the check as it wasn't in a fit to release state, see https://llvm.org/PR49318.
This check has since had some of those issues addressed and should be good for the next release cycle now, pending any more bug reports about it.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D97275