(from #30876)

centos-7 has EOL June 2024 and comes with the very outdated gcc 4.x, which we will not be able to use any more (see #32074)

We add variants centos-7-devtoolset-gcc_{9,10,11}, following https://stackoverflow.com/a/67212990/557937; to test:

tox -e docker-centos-7-devtoolset-gcc_11-standard

This prepares a switch from centos-7 to centos-7-devtoolset-gcc_11, as suggested by dimpase in #30876 comment:12
("nobody nowadays is using gcc 4 on Centos 7... We can and should drop gcc 4, without dropping OSs")

(This is not needed for fedora because even the oldest that we currently support, fedora-26, comes with GCC 7.1.1.)

Depends on #32966
Depends on #33306

CC: @dimpase

Component: porting

Author: Matthias Koeppe

Branch/Commit: d11dd6e

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/32965