Bean Overrides like `@MockitoBean` and `@TestBean` should not be allowed on `static` fields

[sbrannen]

It's currently possible to declare a bean override field as static.

For example, the following currently works in a test class.

@MockitoBean
static ExampleService service;

However, support for static fields was unintentional and does not align with @Autowired semantics where the annotated field cannot be static.

In light of that, we should reject static bean override fields.