Remove Java 7 support

[elefeint]

Follow-up to #893

The core plugin still refers to isSandboxEnforced() which can be deleted entirely for java7 which is not supported (everything with java7 string can be deleted including tests

As a side effect, these flags can be removed:
-Duse_jetty9_runtime=true -D--enable_all_permissions=true
as we are only on jetty9 now, and we do not have a security manager for java8, 11 and 17.

cc: @ludoch -- follow-up issue to remove Java 7 logic.