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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> Not that it's the end of the world if this doesn't get in before the this release, but @sscalpone , exactly what feedback are we wanting before merging?

IIUC, this: https://discourse.llvm.org/t/proposal-rename-flang-new-to-flang/69462/55

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