Change defaultapp to dashboard

Result of discussion at https://help.nextcloud.com/t/enable-dashboard-on-per-user-basis/94872

Code reference:
https://github.com/nextcloud/server/blob/db86bea18ce41ad73e9c1a06f2b2d89d8d4f2ef8/lib/private/legacy/OC_Util.php#L1106



Signed-off-by: rakekniven <mark.ziegler@rakekniven.de>