datetime.utcfromtimestamp() and .utcnow() are deprecated

[ThomasWaldmann]

src/moin/storage/middleware/indexing.py:177: DeprecationWarning: 
datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in
a future version.

Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    doc[key] = datetime.datetime.utcfromtimestamp(doc[key])

Notable:

• it recommends using datetime.fromtimestamp instead of .utcfromtimestamp .
• it also recommends using timezone-aware datetime objects instead of naive datetime objects - but this is a bigger change and shall not be in the scope of this ticket! I filed do a global review of timestamps, migrate to tz-aware timestamps #1676 for that topic.

https://docs.python.org/3/library/datetime.html#datetime.datetime.fromtimestamp

[ThomasWaldmann]

This is a 100% compatible replacement (we used that in borgbackup to avoid the deprecation warning):

def utcfromtimestamp(timestamp):
    """Returns a naive datetime instance representing the timestamp in the UTC timezone"""
    return datetime.fromtimestamp(timestamp, timezone.utc).replace(tzinfo=None)