Index @ConfigurationProperties so they can be found by scanning

Previously, @ConfigurationProperties was not annotated with @Indexed.
This meant that @ConfigurationPropertiesScan would not be able to
find them when the underlying
ClassPathScanningCandidateComponentProvider is using a
CandidateComponentsIndex.

This commit annotated @ConfigurationProperties with @Indexed so that
they can be found by index-based scanning.

Fixes gh-26459

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/ConfigurationProperties.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,7 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 import org.springframework.core.annotation.AliasFor;
+import org.springframework.stereotype.Indexed;
 
 /**
  * Annotation for externalized configuration. Add this to a class definition or a
@@ -46,6 +47,7 @@ import org.springframework.core.annotation.AliasFor;
 @Target({ ElementType.TYPE, ElementType.METHOD })
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
+@Indexed
 public @interface ConfigurationProperties {
 
 	/**