CI: Remove lingering stdout_callback=yaml in macOS jobs

Support removed in Ansible 13 (ansible-core 2.20).
refs #1285, #1291

tests/image_prep/ansible.cfg

@@ -1,12 +1,13 @@
 
 [defaults]
+# Ansible >= 6 (ansible-core >= 2.13)
+callback_result_format = yaml
 deprecation_warnings = false
 strategy_plugins = ../../ansible_mitogen/plugins/strategy
 retry_files_enabled = false
 display_args_to_stdout = True
 no_target_syslog = True
 host_key_checking = False
-stdout_callback = yaml
 
 [inventory]
 unparsed_is_fatal = true

tox.ini

@@ -117,6 +117,7 @@ setenv =
     PIP_CONSTRAINT={toxinidir}/tests/constraints.txt
     # Superceded in Ansible >= 6 (ansible-core >= 2.13) by result_format=yaml
     # Deprecated in Ansible 12 (ansible-core 2.19)
+    # Removed in Ansible 13 (ansible-core 2.20)
     ans{2.10,3,4,5}: ANSIBLE_STDOUT_CALLBACK=yaml
     # Print warning on the first occurence at each module:linenno in Mitogen. Available Python 2.7, 3.2+.
     PYTHONWARNINGS=default:::ansible_mitogen,default:::mitogen