invalid timestamps in core document properties

Openpyxl currently writes timestamp strings like 2023-12-11T19:03:54+00:00Z into the document properties in docProps/core.xml. The presence of both numerical time zone '+00:00' and UTC indicator 'Z' makes this invalid. This behavior was introduced in !435 (merged).

datetime.datetime.utcnow() and datetime.datetime.now(tz=datetime.timezone.utc) are not the same; the former returns a naive datetime while the latter returns a timezone-aware one. This resulted in a test failure that was also "fixed" in !435 (merged). However, it was not noticed that the code writing the document properties in openpyxl/packaging/core.py#L33 is assuming a naive datetime.

To fix this you could either use datetime.datetime.now(tz=datetime.timezone.utc).replace(tzinfo=None), or drop the + 'Z' from openpyxl/packaging/core.py#L33, or teach the NestedDateTime class to cope with both naive and aware datetimes. (Excel writes the timestamps with UTC timezone and using the 'Z' timezone indicator.)