MINOR: Embedded connect cluster should mask exit procedures by default (#7028)

`EmbeddedConnectCluster` has the ability to mask system exits to avoid killing the jvm. It appears that the default was intended to be `true`, but is actually `false`. The `maskExitProcedures` method on `EmbeddedConnectCluster.Builder` documents the parameter as:

```
* @param mask if false, exit and halt procedures remain unchanged; true is the default.
```
Because this is not enabled by default as intended, we are seeing some build failures which exit abruptly:
```
17:29:11 Execution failed for task ':connect:runtime:integrationTest'.
17:29:11 > Process 'Gradle Test Executor 25' finished with non-zero exit value 1
```
The culprit often appears to be `ExampleConnectIntegrationTest`, which indeed does not override the default value of `maskExitProcedures`.

Reviewers: Ewen Cheslack-Postava <me@ewencp.org>

connect/runtime/src/test/java/org/apache/kafka/connect/util/clusters/EmbeddedConnectCluster.java

@@ -429,7 +429,7 @@ public class EmbeddedConnectCluster {
         private int numWorkers = DEFAULT_NUM_WORKERS;
         private int numBrokers = DEFAULT_NUM_BROKERS;
         private Properties brokerProps = DEFAULT_BROKER_CONFIG;
-        private boolean maskExitProcedures = false;
+        private boolean maskExitProcedures = true;
 
         public Builder name(String name) {
             this.name = name;