compilers: add comment on older GCC usage

This provides some context on why we may want to keep some of the older
GCC versions around. These are not hard dates for us so can still remove
support for older versions if there is a reason to do so.

References:
- https://documentation.ubuntu.com/ubuntu-for-developers/reference/availability/gcc/
- https://endoflife.date/ubuntu
- https://access.redhat.com/solutions/19458
- https://endoflife.date/rhel