Upgrade to Gradle 6.9

Closes gh-26408

gradle/wrapper/gradle-wrapper.properties

 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.8.3-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.9-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/junit/GradleCompatibilityExtension.java

@@ -45,10 +45,10 @@ public final class GradleCompatibilityExtension implements TestTemplateInvocatio
 		JavaVersion javaVersion = JavaVersion.current();
 		if (javaVersion.isCompatibleWith(JavaVersion.VERSION_14)
 				|| javaVersion.isCompatibleWith(JavaVersion.VERSION_13)) {
-			GRADLE_VERSIONS = Arrays.asList("6.3", "6.4.1", "6.5.1", "6.6.1", "6.7.1", "default", "6.9-rc-2");
+			GRADLE_VERSIONS = Arrays.asList("6.3", "6.4.1", "6.5.1", "6.6.1", "6.7.1", "6.8.3", "default");
 		}
 		else {
-			GRADLE_VERSIONS = Arrays.asList("5.6.4", "6.3", "6.4.1", "6.5.1", "6.6.1", "6.7.1", "default", "6.9-rc-2");
+			GRADLE_VERSIONS = Arrays.asList("5.6.4", "6.3", "6.4.1", "6.5.1", "6.6.1", "6.7.1", "6.8.3", "default");
 		}
 	}