ci: Upgrade FreeBSD image

Upgrade to the current stable version. To avoid needing commits like this in
the future, the CI image name now doesn't contain the OS version number
anymore.

Backpatch to all versions with CI support, we don't want to generate CI images
for multiple FreeBSD versions.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAN55FZ3_P4JJ6tWZafjf-_XbHgG6DQGXhH-y6Yp78_bwBJjcww@mail.gmail.com

.cirrus.tasks.yml

@@ -32,7 +32,7 @@ on_failure: &on_failure
     type: text/plain
 
 task:
-  name: FreeBSD - 13
+  name: FreeBSD
 
   env:
     # FreeBSD on GCP is slow when running with larger number of CPUS /
@@ -40,7 +40,7 @@ task:
     CPUS: 2
     BUILD_JOBS: 3
     TEST_JOBS: 3
-    IMAGE_FAMILY: pg-ci-freebsd-13
+    IMAGE_FAMILY: pg-ci-freebsd
     DISK_SIZE: 50
 
     CCACHE_DIR: /tmp/ccache_dir