[clang] [flang] [flang][driver] Rename `flang-new` as `flang` (PR #74377)

[Thorsten Schütt via cfe-commits]

tschuett wrote:

Are you going to add an entry to the [Release Nodes](https://github.com/llvm/llvm-project/blob/main/flang/docs/ReleaseNotes.md)?

https://github.com/llvm/llvm-project/pull/74377