applets/digitalclock: make tooltip and multi-timezone follow locale date format

5a32ea2b move Qt.formatDate usage to Qt6 new way, but left code in Tooltip
and timezone. This commit pass Qt.locale to Qt.formatDate in tooltip and
multi-timezone, make sure them follow locale date format.

BUG: 477594


FIXED-IN: 6.0

Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
(cherry picked from commit b2116d1f)