fix: use daemon threads internally (#2118)

* chore(csql): use daemon threads internally

Author: kgala2

core/src/main/java/com/google/cloud/sql/core/InternalConnectorRegistry.java

@@ -34,7 +34,6 @@
 import java.util.Properties;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executors;
-import java.util.concurrent.ScheduledThreadPoolExecutor;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -147,12 +146,14 @@ static ListeningScheduledExecutorService getDefaultExecutor() {
     // configure 3 or fewer instances, requiring 6 threads during refresh. By setting
     // this to 8, it's enough threads for most users, plus a safety factor of 2.
 
-    ScheduledThreadPoolExecutor executor =
-        (ScheduledThreadPoolExecutor) Executors.newScheduledThreadPool(8);
-
-    executor.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
     return MoreExecutors.listeningDecorator(
-        MoreExecutors.getExitingScheduledExecutorService(executor));
+        Executors.newScheduledThreadPool(
+            8,
+            r -> {
+              Thread t = new Thread(r);
+              t.setDaemon(true);
+              return t;
+            }));
   }
 
   /**