SUPPORT.md: limit security support for hosts with very much memory

SUPPORT.md: limit security support for hosts with very much memory

Sufficient and in particular regular testing on very large hosts cannot
currently be guaranteed. Anyone wanting us to support larger hosts is
free to propose so, but will need to supply not only test results, but
also a test plan.

This is a follow-up to XSA-385.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>