AnnotationTypeMapping tracks @AliasFor mappings as convention-based

[sbrannen]

While working on #28760, I noticed that AnnotationTypeMapping.addConventionMappings() actually adds convention-based mappings that it should not.

For example, in certain circumstances an explicit annotation attribute override configured via @AliasFor can be mapped as convention-based.

Although this does not appear to cause negative side effects (other than unnecessary processing), this is technically a bug that should be addressed.

However, since there may be unknown use cases that somehow rely on the behavior of this bug, I currently only intend to apply the fix to 6.0.