Slow GCS uploads for large Java staging jars

The default and max upload chunk size is 1M, in 

[https://github.com/apache/beam/blob/master/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/util/GcsStager.java#L96](https://github.com/apache/beam/blob/master/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/util/GcsStager.java#L96)

This should be at least 8M to improve performance.

8m is the default python write chunk size

[https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/gcsio.py#L86](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/gcsio.py#L86)

Imported from Jira [BEAM-8832](https://issues.apache.org/jira/browse/BEAM-8832). Original Jira may contain additional context.
Reported by: amyrvold.