[flang] Relax the Bash version check

As per https://github.com/flang-compiler/f18-llvm-project/issues/1344,
the `flang` bash script works fine with 4.4.19 and requiring
4.4.23 is too restrictive. Rather than keep updating the patch level,
this patch removes this particular check (so that it will only check the
major and minor versions instead).

As this is both rather straightforward and urgent, I'm merging this
without a review.