Document support for Gradle 6.9

Closes gh-26409

spring-boot-project/spring-boot-docs/src/docs/asciidoc/build-tool-plugins/gradle.adoc

 [[build-tool-plugins.gradle]]
 == Spring Boot Gradle Plugin
 The Spring Boot Gradle Plugin provides Spring Boot support in Gradle, letting you package executable jar or war archives, run Spring Boot applications, and use the dependency management provided by `spring-boot-dependencies`.
-It requires Gradle 6.8 or 7.x.
+It requires Gradle 6.8, 6.9, or 7.x.
 Please refer to the plugin's documentation to learn more:
 
 * Reference ({spring-boot-gradle-plugin-docs}[HTML] and {spring-boot-gradle-plugin-pdfdocs}[PDF])

spring-boot-project/spring-boot-docs/src/docs/asciidoc/getting-started/installing.adoc

@@ -45,7 +45,7 @@ More details on getting started with Spring Boot and Maven can be found in the {
 
 [[getting-started.installing.java.gradle]]
 ==== Gradle Installation
-Spring Boot is compatible with Gradle 6.8 and 7.x.
+Spring Boot is compatible with Gradle 6.8, 6.9, and 7.x.
 If you do not already have Gradle installed, you can follow the instructions at https://gradle.org.
 
 Spring Boot dependencies can be declared by using the `org.springframework.boot` `group`.

spring-boot-project/spring-boot-docs/src/docs/asciidoc/getting-started/system-requirements.adoc

@@ -12,7 +12,7 @@ Explicit build support is provided for the following build tools:
 | 3.5+
 
 | Gradle
-| 6.8.x and 7.x
+| 6.8.x, 6.9.x, and 7.x
 |===
 
 

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

@@ -42,7 +42,7 @@ Andy Wilkinson, Scott Frederick
 == Introduction
 The Spring Boot Gradle Plugin provides Spring Boot support in https://gradle.org[Gradle].
 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 6.8 or 7.x and can be used with Gradle's {gradle-userguide}/configuration_cache.html[configuration cache].
+Spring Boot's Gradle plugin requires Gradle 6.8, 6.9, or 7.x and can be used with Gradle's {gradle-userguide}/configuration_cache.html[configuration cache].
 
 In addition to this user guide, {api-documentation}[API documentation] is also available.