[FLINK-12963] [state-processor] Make OperatorTransformation non-insta…

…ntiable

The OperatorTransformation class is a simple entry point for creating
BootstrapTransformations via the #bootstrapWith static factory method.
This commit solidifies this by making the class non-instantiable.

This closes apache#8861.

flink-libraries/flink-state-processing-api/src/main/java/org/apache/flink/state/api/OperatorTransformation.java

@@ -52,7 +52,9 @@
  */
 @PublicEvolving
 @SuppressWarnings("WeakerAccess")
-public abstract class OperatorTransformation {
+public final class OperatorTransformation {
+
+	private OperatorTransformation() {}
 
 	/**
 	 * Create a new {@link OperatorTransformation} from a {@link DataSet}.