[stable-2.6] Keep existing to_yaml behavior with pyyaml >= 5.1. (#53772)

In pyyaml versions before 5.1 the default_flow_style for yaml.dump
was None. Starting with 5.1 it is now False. This change explicitly
sets the value to None to maintain the original to_yaml behavior.

The change to pyyaml was made in the following commit:

507a464ce6
(cherry picked from commit 7f0e09aa31)

Co-authored-by: Matt Clay <matt@mystile.com>

changelogs/fragments/to_yaml-default_flow_style.yaml

@@ -0,0 +1,2 @@
+minor_changes:
+  - "``to_yaml`` filter updated to maintain formatting consistency when used with ``pyyaml`` versions 5.1 and later (https://github.com/ansible/ansible/pull/53772)"

lib/ansible/plugins/filter/core.py

@@ -62,7 +62,8 @@ UUID_NAMESPACE_ANSIBLE = uuid.UUID('361E6D51-FAEC-444A-9079-341386DA8E2E')
 
 def to_yaml(a, *args, **kw):
     '''Make verbose, human readable yaml'''
-    transformed = yaml.dump(a, Dumper=AnsibleDumper, allow_unicode=True, **kw)
+    default_flow_style = kw.pop('default_flow_style', None)
+    transformed = yaml.dump(a, Dumper=AnsibleDumper, allow_unicode=True, default_flow_style=default_flow_style, **kw)
     return to_text(transformed)