DGC-7: Remove default value for "Conference year" field

[maxstarkenburg]

The default was set to "2023", so editors weren't being vigilant about changing it, so the URL path wasn't getting updated. These pages can all be re-saved with the "Conference year" field set to "2024" in order for the URL path to get updated (the old URLs will redirect to the new one):

This PR removes the default value, forcing content editors to choose an appropriate value when creating an announcement:

[mikemadison13]

@maxstarkenburg is there a reason you wouldn't set this to default to the current year? that's the normal behavior.

[maxstarkenburg]

Hi @mikemadison13, yes, precisely to avoid the same problem next year (and the year after that, and ...) that this ticket is meant to resolve (i.e. 2024's announcements being set to 2023, because content editors weren't updating that field value (seemingly glancing over this field, and/or thinking the path alias required a whole ticket to resolve)).

Unless we have an annually recurring ticket to update this, this feels like the "safer" option to keep the alias correct IMO.