Update common.cfg (#4705)
Update common.cfg (#4705)

Java 8 continuous builds are timing out at 5h (300m), increase to 6h
GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples