libtracker-extract: Use only date bits in album/disc URN
This makes the URNs more readable.
Before:
urn:album:awesome%20album:my%20album%20artist:2020-01-01T00:00:00+00:00
After:
urn:album:awesome%20album:my%20album%20artist:2020-01-01
This is an API change.