Remove `@FunctionalInterface` declaration from `BeanOverrideProcessor`

[sbrannen]

A BeanOverrideProcessor is always instantiated via reflection and is never supplied as a lambda expression. In addition, we may later need to introduce additional methods in the BeanOverrideProcessor interface (which is the case for #33925, even though that new method will be a default method).

In light of that, we should remove the @FunctionalInterface declaration from BeanOverrideProcessor.