[goal] Deprecate the JSON formatted policy file

As per the community goal of migrating the policy file
the format from JSON to YAML[1], we need to do two things:

1. Change the default value of '[oslo_policy] policy_file''
config option from 'policy.json' to 'policy.yaml' with
upgrade checks.

2. Deprecate the JSON formatted policy file on the project side
via warning in doc and releasenotes.

Also replace policy.json to policy.yaml ref from doc and tests.

[1]https://governance.openstack.org/tc/goals/selected/wallaby/migrate-policy-format-from-json-to-yaml.html

Change-Id: I207c02ba71fe60635fd3406c9c9364c11f259bae

releasenotes/notes/deprecate-json-formatted-policy-file-3a92379e9f5dd203.yaml

@@ -0,0 +1,20 @@
+---
+upgrade:
+  - |
+    The default value of ``[oslo_policy] policy_file`` config option has
+    been changed from ``policy.json`` to ``policy.yaml``.
+    Operators who are utilizing customized or previously generated
+    static policy JSON files (which are not needed by default), should
+    generate new policy files or convert them in YAML format. Use the
+    `oslopolicy-convert-json-to-yaml
+    <https://docs.openstack.org/oslo.policy/latest/cli/oslopolicy-convert-json-to-yaml.html>`_
+    tool to convert a JSON to YAML formatted policy file in
+    backward compatible way.
+deprecations:
+  - |
+    Use of JSON policy files was deprecated by the ``oslo.policy`` library
+    during the Victoria development cycle. As a result, this deprecation is
+    being noted in the Wallaby cycle with an anticipated future removal of support
+    by ``oslo.policy``. As such operators will need to convert to YAML policy
+    files. Please see the upgrade notes for details on migration of any
+    custom policy files.