Change the datetime service in tests/modules/automatic_updates_test/automatic_updates_test.services.yml to a decorator

Problem/Motivation

In #3236299: User should not be allowed to do update if it is from minor version to another minor version we found that the datetime service in tests/modules/automatic_updates_test/automatic_updates_test.services.yml is not overriding the one we want it to override instead it is overriding the one from the update_test module as we are installing this module in in our ReadinessValidationTest.

Proposed resolution

Change the datetime service in tests/modules/automatic_updates_test/automatic_updates_test.services.yml to a decorator.