lang/gcc9-devel: Relinquish maintainership

After working on (and largely maintaining) our GCC ports for more than
19 years, time has come to hand over the baton. Sadly despite multiple
requests nobody stepped up, so return this port to the pool.

Still happy to provide guidance and a helping hand, for example working
with upstream or on operating the (crucial) nightly testers I have been
running.