Deprecate auth_version option

The same option in tempest was deprecated by [1], because Identity v3
API is the only available version now. Deprecate the option and remove
the old logic to support Identity v2 API which was already removed.

[1] https://review.opendev.org/c/openstack/tempest/+/906158

Change-Id: Id12672fabfd51755eeff7b0d701abfb63e0abce0
diff --git a/releasenotes/notes/deprecate-auth_version-45a3cbb9711ac0aa.yaml b/releasenotes/notes/deprecate-auth_version-45a3cbb9711ac0aa.yaml
new file mode 100644
index 0000000..206edad
--- /dev/null
+++ b/releasenotes/notes/deprecate-auth_version-45a3cbb9711ac0aa.yaml
@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    The ``auth_version`` option has been deprecated and has no effect. This
+    option will be removed in a future release.