Add migration note about using @ConfigMapping only for Interfaces

[Sgitario]

Description

In 483c4ff, we upgraded SmallRye Config to 2.4.3 and it seems that there is no longer possible to use the annotation @ConfigMapping on classes, only in interfaces.

Therefore, users that were using:

@ConfigMapping(prefix = "person")
public class Person {
    String name;
    int age;
}

Before, it worked fine, but now it throws an exception at startup:

Error:  Failed to execute goal io.quarkus:quarkus-maven-plugin:999-SNAPSHOT:build (build) on project properties: Failed to build quarkus application: io.quarkus.builder.BuildException: Build failure: Build failed due to errors
Error: r]: Build step io.quarkus.arc.deployment.ConfigBuildStep#generateConfigClasses threw an exception: java.lang.IllegalStateException: SRCFG00043: The @ConfigMapping annotation can only be placed in interfaces, class io.quarkus.qe.properties.configmapping.Person is a class
Error:  	at io.smallrye.config.ConfigMappingLoader.validateAnnotations(ConfigMappingLoader.java:95)
Error:  	at io.smallrye.config.ConfigMappingLoader.getConfigMappingClass(ConfigMappingLoader.java:44)
Error:  	at io.smallrye.config.ConfigMappingLoader.getConfigMappingInterface(ConfigMappingLoader.java:40)
Error:  	at io.smallrye.config.ConfigMappingLoader.getConfigMappingsMetadata(ConfigMappingLoader.java:34)
Error:  	at io.quarkus.arc.deployment.ConfigBuildStep.generateConfigClasses(ConfigBuildStep.java:289)

I think this should be noted in the migration guide for 2.2.

Implementation ideas

No response

[rsvoboda]

CC @radcortez

[geoand]

Done: https://github.com/quarkusio/quarkus/wiki/Migration-Guide-2.2#smallrye-config