Eliminate the use of deprecated utcnow()

[downtownallday]

Starting In Python3.12, a deprecation warning is printed when calling datetime.datetime.utcnow():

management/ssl_certificates.py:103: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC)

Both MIAB-LDAP and MIAB only run on Ubuntu 22 (Jammy), which has Python 3.10, and does not receive this warning message. However, the code is shared with the cloud-in-a-box project, which runs on Ubuntu 24 (Noble) and does gets this message so am fixing it here, at the "source".