illegal signature attribute when compiling with JDK 11

The following code is invalid, and fails to compile with the JDK 11 javac: https://github.com/apache/beam/blob/1cbbc4d8755130f137e7d9ef4813e8d3b917cb70/sdks/java/harness/src/main/java/org/apache/beam/fn/harness/WindowMergingFnRunner.java#L132-L144

The error is:

```
error: error while generating class <anonymous org.apache.beam.fn.harness.WindowMergingFnRunner$MergingViaWindowFnRunner$1>
(illegal signature attribute for type CAP#1)
where CAP#1 is a fresh type-variable:
CAP#1 extends Object from capture of ?
```

The code was previously accepted by javac due to a bug, which has been fixed in JDK 11: https://bugs.openjdk.java.net/browse/JDK-8203436