Update feed only if nextUpdateTime has been reached

[Grotax]

Summary

This changes the logic of updating/fetching a feed to check for the nextUpdateTime before the the update is executed.

If that time is not reached yet the feed is not updated. This is mainly a advantage for feeds that are updated rarely and will therefore receive less requests. For the nextcloud instance this is also a benefit because feed updates that are likely not needed are skipped, which saves compute time.

Depends on #2993

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Changelog entry added for all important changes.

[Grotax]

I'm currently struggling a bit with this for multiple reasons.

1. In my dev environment nextUpdateTime in the DB ends up being Null for some reason for some feeds while in the debug log I see that the nextUpdateTime was logged
2. Also in the Updater test that I added the field of that feed is null after adding it which i expected but then it stays null after running an update it should no longer be null
3. If it was working I'm not sure how to test the update behavior since the date would be in one hour, because feed-io sets that as by default as the smallest nextUpdateTime which i think makes sense for real world. But for most of the updater tests that would mean they break.

Anyway can someone tell me why the field can end up being null?