docs: simplify and clarify the platform support rules

docs: simplify and clarify the platform support rules

The distinction between short life and long life Linux distributions
turned out to be redundant. They can both be covered in a simple way
by noting support will target the current release, and the previous
release for a period of two years or until its EOL. This rule can also
apply to the other UNIX based distros, leaving only Windows needing a
different set of rules.

This also clarifies that Debian LTS is out of scope, because the LTS
support is provided by a separate group from the main Debian maintainer
team.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>