[dhcp_relay] Update separator for key in COUNTERS_DB for per-interface counter (sonic-net#22186)

Why I did it
Per database_config.json.j2, separator in COUNTERS_DB should be ":"
PR in dhcpmon has been merged: sonic-net/sonic-dhcpmon#38

How I did it
Update separator to ":"

How to verify it
Manually test

Author: yaqiangz

dockers/docker-dhcp-relay/start.sh

@@ -10,7 +10,7 @@ then
     ${CTR_SCRIPT} -f dhcp_relay -o ${RUNTIME_OWNER} -v ${IMAGE_VERSION}
 fi
 
-keys=$(sonic-db-cli COUNTERS_DB keys "DHCPV4_COUNTER_TABLE|*")
+keys=$(sonic-db-cli COUNTERS_DB keys "DHCPV4_COUNTER_TABLE:*")
 for key in $keys; do
     sonic-db-cli COUNTERS_DB del "$key"
 done