[flang-dev] Replacing `f18` with `flang-new`

[Andrzej Warzynski via flang-dev]

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] https://reviews.llvm.org/D103177
[2] https://github.com/llvm/llvm-project/blob/main/flang/tools/f18/flang