Recommend the use of GCC 8 (instead of GCC 7 until now) to replace
these older versions of GCC - GCC 4.8, GCC 5, and GCC 6 - which have
been end-of-lifed upstream many moons ago. GCC 8 has been the default
version of GCC in the Ports Collection for a while and as such proven
itself, plus of all versions it is most likely to be present/used.

No functional change, just updated advice to our users.