MINOR: suppress unchecked warning in WorkerTest (#23893)

Mocking `Transformation.class` with Mockito returns a raw type, causing
an unchecked conversion to `Transformation<SourceRecord>`.

```
> Task :connect:runtime:compileTestJava
Note:
/Users/parker/opensource/kafka/connect/runtime/src/test/java/org/apache/kafka/connect/runtime/WorkerTest.java
uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
```

Add a local `@SuppressWarnings("unchecked")`, following the existing
approach in `TransformationStageTest.applyAndAssert`.

Validation:
- `./gradlew :connect:runtime:compileTestJava --rerun-tasks`
- The unchecked message for WorkerTest.java no longer appeared.

Reviewers: Ken Huang <s7133700@gmail.com>, Chia-Ping Tsai
 <chia7712@gmail.com>

Author: Parkerhiphop

connect/runtime/src/test/java/org/apache/kafka/connect/runtime/WorkerTest.java

@@ -975,6 +975,7 @@ public void testStartTaskFailureAfterConverterWrappingClosesConverterPlugins(boo
         mockVersionedTaskConverterFromConnector(ConnectorConfig.VALUE_CONVERTER_CLASS_CONFIG, ConnectorConfig.VALUE_CONVERTER_VERSION_CONFIG, taskValueConverter);
         mockVersionedTaskHeaderConverterFromConnector(taskHeaderConverter);
 
+        @SuppressWarnings("unchecked")
         Transformation<SourceRecord> brokenTransform = mock(Transformation.class);
         when(brokenTransform.config()).thenReturn(new ConfigDef());
         doThrow(new RuntimeException("Failed to configure transform")).when(brokenTransform).configure(any());