Some datetimes end with ZZ

[pjhartzell]

Describe the bug

The occasional start_datetime, end_datetime, or plain datetime ends with a double Z, e.g., 2023-06-13T11:33:10ZZ. This also applies to datetimes that are nested in custom fields, e.g.:

"s5p:o3_tcl": {
    "shape_ccd": [
        80,
        360
    ],
    "shape_csa": [
        8,
        18
    ],
    "troposphere_end_datetime": "2023-06-17T23:48ZZ",
    "stratosphere_end_datetime": "2023-06-19T12:18:48ZZ",
    "troposphere_start_datetime": "2023-06-14T23:04ZZ",
    "stratosphere_start_datetime": "2023-06-13T11:33:10ZZ"

Note the above example of nested datetimes is truncated for brevity.

To reproduce

Create an Item that happens to have some erroneous metadata.