Fix type of param for creation column

[tcitworld]

It's actually an int, not a date type.

This is the only use of limitToDBFieldDateTime so I change it directly in the function, but could be troublesome later.

Closes https://github.com/nextcloud/files_lock/issues/15#issuecomment-631456327

[juliusknorr]

Thanks, I pushed a separate commit to actually migrate away from the abstraction bits and just use the regular query builder for that simple filtering.