cirrus.yml: Update the macOS jobs to Catalina

When looking at the CI jobs on cirrus-ci.com, it seems like the mojave-based
images have been decomissioned a while ago already, since apparently all our
jobs get automatically upgraded to catalina. So let's update our YML script
accordingly to avoid confusion.

Reviewed-by: Ed Maste <emaste@freebsd.org>
Message-Id: <20200728074405.13118-5-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.cirrus.yml

@@ -18,7 +18,7 @@ freebsd_12_task:
 
 macos_task:
   osx_instance:
-    image: mojave-base
+    image: catalina-base
   install_script:
     - brew install pkg-config python gnu-sed glib pixman make sdl2 bash
   script:
@@ -33,7 +33,7 @@ macos_task:
 macos_xcode_task:
   osx_instance:
     # this is an alias for the latest Xcode
-    image: mojave-xcode
+    image: catalina-xcode
   install_script:
     - brew install pkg-config gnu-sed glib pixman make sdl2 bash
   script: