Convert newly added config to @ConfigMapping

(cherry picked from commit b7f641c)

core/runtime/src/main/java/io/quarkus/runtime/BuilderConfig.java

@@ -2,28 +2,27 @@
 
 import java.util.Optional;
 
-import io.quarkus.runtime.annotations.ConfigItem;
 import io.quarkus.runtime.annotations.ConfigPhase;
 import io.quarkus.runtime.annotations.ConfigRoot;
+import io.smallrye.config.ConfigMapping;
 
 /**
  * This configuration class is here to avoid warnings when using {@code -Dquarkus.builder.=...}.
  *
  * @see io.quarkus.builder.BuildChainBuilder
  */
-@ConfigRoot(name = "builder", phase = ConfigPhase.RUN_TIME)
-public class BuilderConfig {
+@ConfigMapping(prefix = "quarkus.builder")
+@ConfigRoot(phase = ConfigPhase.RUN_TIME)
+public interface BuilderConfig {
 
  /**
  * Dump the graph output to a file. This is useful for debugging.
  */
- @ConfigItem
- public Optional<String> graphOutput;
+ Optional<String> graphOutput();
 
  /**
  * Whether to log the cause of a conflict.
  */
- @ConfigItem
- public Optional<Boolean> logConflictCause;
+ Optional<Boolean> logConflictCause();
 
 }