cirrus-ci: update FreeBSD versions

FreeBSD 14.0 has already been released, so switch to the release version image,
and introduce a FreeBSD 15.0 version to track current FreeBSD unstable
(development) branch.

Sample output at:

https://github.com/royger/xen/runs/19105278189

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

.cirrus.yml

@@ -29,5 +29,11 @@ task:
 task:
   name: 'FreeBSD 14'
   freebsd_instance:
-    image_family: freebsd-14-0-snap
+    image_family: freebsd-14-0
+  << : *FREEBSD_TEMPLATE
+
+task:
+  name: 'FreeBSD 15'
+  freebsd_instance:
+    image_family: freebsd-15-0-snap
   << : *FREEBSD_TEMPLATE