Use the new equivalent updates API to prevent updates from 10.4.0 to 11.0.0

So ran the test locally without the change to system.install and the test still passes. Shouldn't it fail?

No it should not fail, it is independent of the system.install. The test is relying on the downgrade_test module to provide the hook and that module is only installed in the test after proving the requirement failure. The test is a functional test of the concept. There is no similar test using the actual system_update_11101 because I don't see a way to prevent the update process from finding and executing that hook.

I left some comments on the test. It could use some clarification since update tests are dark magic.

I think we need also need the MR that adds the equivalent system update to review this properly, no?

Still NW for a second MR for 10.4, I think.