lang/gcc10-devel: Deprecate port

The purpose of lang/gcc10-devel and other lang/gcc*-devel ports
was to track GCC's weekly snapshots, serve as a canary for the
next update of the corresponding lang/gcc* port that tracks minor
releases, and provide bleeding edge for users who desire so.

Since I relinquished maintainership more than five months ago
(commit 29510fc89b7e40ee7c918564ad2173b70e98c4ee) there has not
been an update and the port is stuck at the 20211008 snapshot.

It thus has not been serving its purpose any longer for a while
and GCC 10 is in a rather stable state, so deprecate the port for
removal more than half a year after the latest version update.