docs/about/build-platforms: Refine the distro support policy

docs/about/build-platforms: Refine the distro support policy

For long-term distributions that release a new version only very
seldom, we limit the support to five years after the initial release.
Otherwise, we might need to support distros like openSUSE 15 for
up to 7 or even more years in total due to our "two more years
after the next major release" rule, which is just way too much to
handle in a project like QEMU that only has limited human resources.

Message-Id: <20230223193257.1068205-1-thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>