Replacing `f18` with `flang-new`

Hello,

As mentioned in our community call yesterday, once [1] is merged, `flang-new` will be ready to replace `f18`. I propose that it is deleted soon after so that we have only 1 driver to maintain.

As suggested by Peter Steinfeld, let's have a transition period of at least 2 weeks. I suggest that it's 2 weeks from when [1] is merged. Hopefully this will be sufficient for you to adjust your workflows. If not, we will extend this.

Please, if you either:
* use `flang` (the bash script [2]) in your workflows, or
* rely on `f18`s ability to call external compilers,
could you check whether [1] preservers everything that you need? With no testing upstream, it's been tricky to verify.

Thank you,
-Andrzej

[1] ⚙ D103177 [flang][driver] Extend the `flang` bash script to act as a driver
[2] llvm-project/flang at main · llvm/llvm-project · GitHub

Just a quick update, I've just uploaded a patch to delete `f18`: ⚙ D105811 [flang][driver] Delete `f18` (i.e. the old Flang driver)

-Andrzej