SUPPORT.md: split XSM from Flask

SUPPORT.md: split XSM from Flask

XSM is a generic framework, which in particular is also used by SILO.
With this it can't really be experimental: Arm mandates SILO for having
a security supported configuration.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>
master commit: d7c18b8720824d7efc39ffa7296751e1812865a9
master date: 2024-09-04 16:05:03 +0200