python 3.12: utcnow function is deprecated

The following warning is displayed on a system running with Python 3.12:
-------------------
/usr/lib/python3.12/site-packages/ipalib/rpc.py:925: DeprecationWarning:
datetime.utcnow() is deprecated and scheduled for removal in a future
version. Use timezone-aware objects to represent datetimes in UTC:
datetime.now(datetime.UTC).

  timestamp=datetime.datetime.utcnow())
-------------------

Fixes: https://pagure.io/freeipa/issue/9425
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>

ipatests/create_external_ca.py

@@ -35,7 +35,7 @@ class ExternalCA:
     """
 
     def __init__(self, days=365, key_size=None):
-        self.now = datetime.datetime.utcnow()
+        self.now = datetime.datetime.now(tz=datetime.UTC)
         self.delta = datetime.timedelta(days=days)
         self.ca_key = None
         self.ca_public_key = None