CI: Only use Sonoma for macOS

.cirrus.yml

@@ -161,18 +161,15 @@ alpine_task:
   << : *CI_TEMPLATE
   << : *UNIX_ENV
 
-# Apple doesn't publish official long-term support timelines.
-# We aim to support both the current and previous macOS release.
+# Cirrus only supports the following macos runner currently, selecting
+# anything else automatically upgrades to this one.
+#
+#    ghcr.io/cirruslabs/macos-runner:sonoma
+#
+# See also: https://cirrus-ci.org/guide/macOS/
 macos_sonoma_task:
   macos_instance:
-    image: ghcr.io/cirruslabs/macos-sonoma-base:latest
-  prepare_script: ./ci/macos/prepare.sh
-  << : *CI_TEMPLATE
-  << : *MACOS_RESOURCES_TEMPLATE
-
-macos_ventura_task:
-  macos_instance:
-    image: ghcr.io/cirruslabs/macos-ventura-base:latest
+    image: ghcr.io/cirruslabs/macos-runner:sonoma
   prepare_script: ./ci/macos/prepare.sh
   << : *CI_TEMPLATE
   << : *MACOS_RESOURCES_TEMPLATE