Remove all pre-C11 legacy code paths from the tree

See also !12086 (closed): we already unconditionally use C11 style atomics in numerous places even before that patch, so there's no point to consider support for pre-C11 ancient gcc versions in 2024. We should remove all pre-C11 legacy code paths from the tree, using the oldest distro we currently test in CI as our borderline (CentOS7/gcc-4.8).

Also related: !12810 (closed), more cleanups will be done in another subsequent patch.