PB-1495 Remove date comparison for property expires

[asteiner-swisstopo]

In the forecast extension, datetime is the forecast date. So datetime can be anywhere in the future and expires might be before that. So comparing these two dates makes no sense. Same for end_datetime.

The expires field is part of the Timestamps extension, so it might be used in different contexts where this check would actually make sense. But ultimately, it is the responsibility of the data owner that the data is consistent. So we remove the check to reduce complexity on our side.

ℹ️ The definition of the fields according to the forecast extension docs:

• datetime: REQUIRED. The forecast datetime. It follows the definition in the STAC Common Metdata. If the forecast is not only for a specific instance in time but instead is for a certain period, you should use start_datetime and end_datetime and set datetime either to reflect the start_datetime (recommended) or to null.
• expires: The datetime until the forecast is valid or gets superseded by a new forecast. It follows the definition in the Timestamps Extension.

[boecklic]

instead of just removing the check completely, we could think about adapting it to sth that makes more sense than what we have currently

[boecklic]

we could replace that with a meaningful check: having an expires timestamp in the past doesn't make sense because it would immediately be deleted again, so this is probably a case we don't want to allow.