devel/flang-clang: Deprecate and set expiration date to 2022-09-30

Current version is very outdated and unsupported upstream
Upstream is working to improve Flang compiler included in LLVM

See https://github.com/flang-compiler/flang/wiki for more information

This change was also discussed with bapt

PR: 263810
Approved by: jmd (maintainer timeout, 6+ weeks)