DOC: update INSTALL to reflect the minimum compiler version

DOC: update INSTALL to reflect the minimum compiler version

The mt_list update in 3.1 mandated the support for c11-like atomics that
arrived with gcc-4.7. As such, older versions are no longer supported.
For special cases in single-threaded environments, mt_lists could be
replaced with regular lists but it doesn't seem worth the hassle. It
was verified that gcc 4.7 to 14 and clang 3.0 and 19 do build fine.
That leaves us with 10 years of coverage of compiler versions, which
remains reasonable assuming that users of old ultra-stable systems are
unlikely to upgrade haproxy without touching the rest of the system.

This should be backported to 3.1.