ci: remove minor version from freebsd image name

This way .cirrus.yml does not need to be changed just because freebsd releases
a new minor version.

Discussion: https://postgr.es/m/20220728095704.ryywoaz4dqqrwstc@alap3.anarazel.de
Backpatch: 15-, just like the CI support

.cirrus.yml

@@ -52,7 +52,7 @@ task:
 
   compute_engine_instance:
     image_project: $IMAGE_PROJECT
-    image: family/pg-ci-freebsd-13-1
+    image: family/pg-ci-freebsd-13
     platform: freebsd
     cpu: $CPUS
     memory: 2G