ci: Upgrade macOS version from 12 to 13.

Back-patch to 15, where in-tree CI began.

Author: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/1441145.1675300332%40sss.pgh.pa.us

.cirrus.yml

@@ -221,7 +221,7 @@ task:
 
 
 task:
-  name: macOS - Monterey
+  name: macOS - Ventura
 
   env:
     CPUS: 4 # always get that much for cirrusci macOS instances
@@ -239,7 +239,7 @@ task:
   only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*(macos|darwin|osx).*'
 
   macos_instance:
-    image: ghcr.io/cirruslabs/macos-monterey-base:latest
+    image: ghcr.io/cirruslabs/macos-ventura-base:latest
 
   sysinfo_script: |
     id