Cirrus CI: Update the macOS task image

Based on https://cirrus-ci.org/guide/macOS/.

This should avoid the warning:
Only ghcr.io/cirruslabs/macos-runner:sonoma is allowed. Automatically
upgraded.

Author: fxlb

.cirrus.yml

@@ -44,7 +44,7 @@ macos_task:
   name: macos-aarch64
   only_if: $CIRRUS_BRANCH != 'coverity_scan'
   macos_instance:
-    image: ghcr.io/cirruslabs/macos-ventura-xcode:14.1 # macOS 13 with Xcode 14.1
+    image: ghcr.io/cirruslabs/macos-runner:sonoma # last 3 versions of Xcode
   env:
     MAKEFLAGS: '-j 4' # macOS VMs run on 4 cores
   script: