fix: datetime.datetime.utcnow() is deprecated

[LAKSHJAIN14]

Types of changes

• Hot fix (emergency fix and release)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation (change which affects the documentation site)
• Breaking change (fix or feature that would cause existing functionality to change)
• Release (develop -> main)

Summary

while running tests there are warnings related to Deprecation of datetime.datetime.utcnow()

trestle/tasks/csv_to_oscal_mc.py:66: 16 warnings
  /mnt/d/compliance-trestle/trestle/tasks/csv_to_oscal_mc.py:66: 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).
    timestamp = datetime.datetime.utcnow().replace(microsecond=0).replace(tzinfo=datetime.timezone.utc).isoformat()

tests/trestle/tasks/csv_to_oscal_mc_test.py: 24 warnings
  /mnt/d/compliance-trestle/trestle/tasks/csv_to_oscal_mc.py:127: 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).
    datetime.datetime.utcnow().replace(microsecond=0).replace(tzinfo=datetime.timezone.utc).isoformat()

These changes should fix the issue and running the tests after the changes had no more warnings related to this
@degenaro

[degenaro]

[degenaro]

LGTM