Document support for Gradle's configuration cache

See gh-22922

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/index.adoc

@@ -43,6 +43,7 @@ The Spring Boot Gradle Plugin provides Spring Boot support in https://gradle.org
 It allows you to package executable jar or war archives, run Spring Boot applications, and use the dependency management provided by `spring-boot-dependencies`.
 Spring Boot's Gradle plugin requires Gradle 6 (6.3 or later).
 Gradle 5.6 is also supported but this support is deprecated and will be removed in a future release.
+Gradle's {gradle-userguide}/configuration_cache.html[configuration cache] is supported when using Gradle 6.7 or later.
 
 In addition to this user guide, {api-documentation}[API documentation] is also available.