Restore `kill -9` to launch.script force-stop

Restore the `-9` flag to the send `kill` call which was inadvertently
lost in commit a35a1022.

Closes gh-6223

spring-boot-tools/spring-boot-loader-tools/src/main/resources/org/springframework/boot/loader/tools/launch.script

@@ -214,7 +214,7 @@ do_force_stop() {
   kill -9 "$1" &> /dev/null || { echoRed "Unable to kill process $1"; return 1; }
   for i in $(seq 1 $STOP_WAIT_TIME); do
     isRunning "$1" || { echoGreen "Stopped [$1]"; rm -f "$2"; return 0; }
-    [[ $i -eq STOP_WAIT_TIME/2 ]] && kill "$1" &> /dev/null
+    [[ $i -eq STOP_WAIT_TIME/2 ]] && kill -9 "$1" &> /dev/null
     sleep 1
   done
   echoRed "Unable to kill process $1";