net: dhcpv4: Cancel pending DNS queries on DNS server update

On processing a DNS server option, which re-initializes the DNS resolve
context, also cancel all pending DNS queries before closing the old DNS
resolve context. Otherwise, `z_clock_announce()` will later work on the
re-initialized context once the queries expire and crash because the
reference to the timeout function `query_timeout()` has been cleared.

Signed-off-by: Markus Fuchs <markus.fuchs@de.sauter-bc.com>