[flang][driver] Remove references to the `flang` bash script

This is a follow-up of https://reviews.llvm.org/D125832
(see also https://reviews.llvm.org/D125788 for more context). It simply
removes any remaining references to the `flang` bash script. Note that
that `flang-to-external-fc` remains intact.

This felt worthwhile mentioning in the release notes, which have not
been updated since LLVM 12 (we are approaching LLVM 15 now). I took the
liberty of removing all of the out-dated content and added a note about
the renaming.

Differential Revision: https://reviews.llvm.org/D127094