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

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@@ -15,17 +15,13 @@ local:
 ```
 
 There are two main drivers in Flang:
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h-vetinari wrote:

It might be worth spending a paragraph or section explaining the historical role of `flang-new`, if only so that people understand why it's (eventually) not available anymore.

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