Most of the config keys defined by the `spring-boot-test-autoconfigure`
module can't be overridden because they are mapped with `@PropertyMapping`
on an annotation. It is confusing that such keys are exposed in content
assistance as using them will have no effect.

This commit removes the annotation processor from the build so that the
`@ConfigurationProperties` beans aren't processed anymore. Instead, manual
metadata is written for the two only keys that are effectively used in
regular configuration.

As a result, the `additional-spring-configuration-metadata` file has been
renamed to `spring-configuration-metadata` since nothing is processing it
anymore.

Closes gh-7887