cirrus-ci: introduce FreeBSD 15.0-RELEASE as "current" version

Switch the current version to 15.0-RELEASE.  Sadly the 16 snapshot images
are not working, hence use the FREEBSD_CURRENT variable as a placeholder
for 15.0 until the issues with FreeBSD 16.0 snapshot images is solved.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
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>
(cherry picked from commit cfb767759b)

.cirrus.yml

@@ -3,7 +3,7 @@ freebsd_versions: &FREEBSD_VERSIONS
   env:
     FREEBSD_LEGACY:     freebsd-13-5
     FREEBSD_PRODUCTION: freebsd-14-3
-    FREEBSD_CURRENT:    freebsd-15-0-snap
+    FREEBSD_CURRENT:    freebsd-15-0-amd64-ufs
 
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