build: remove GCC7 support

The development branch is now on version 10, we shouldn't drag to many
old versions and therefore drop at least 7.x.

Signed-off-by: Paul Spooren <mail@aparcar.org>
Acked-by: Rosen Penev <rosenp@gmail.com>
Acked-by: Rui Salvaterra <rsalvaterra@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Aleksander Jan Bajkowski <A.Bajkowski@stud.elka.pw.edu.pl>