Replace dateutils usage with datetime and oslo.utils

This cr fixes:
* Replaced ``dateutil.tz.tzlocal()`` and ``dateutil.tz.tzutc()`` with
  ``datetime.timezone`` built-in classes in audit controllers and
  continuous audit scheduling.

* Replaced ``dateutil.parser.parse()`` with
  ``oslo_utils.timeutils.parse_isotime()`` in the zone migration
  strategy for parsing datetime strings.

Closes-Bug: #2118404

Change-Id: I6d8a345fa4339a688769b147413dcdf3016bf4a0
Signed-off-by: Chandan Kumar (raukadah) <chkumar@redhat.com>

releasenotes/notes/drop-python-dateutil-dependency-2118404-f5a8b2c1e9d4a6b3.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Removed the ``python-dateutil`` dependency from Watcher to reduce the
+    number of external dependencies and improve maintainability.