Test the Gradle plugin against Gradle 6.9-rc-1

Closes gh-26267

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");
+			GRADLE_VERSIONS = Arrays.asList("6.3", "6.4.1", "6.5.1", "6.6.1", "6.7.1", "default", "6.9-rc-1");
 		}
 		else {
-			GRADLE_VERSIONS = Arrays.asList("5.6.4", "6.3", "6.4.1", "6.5.1", "6.6.1", "6.7.1", "default");
+			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-1");
 		}
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/testkit/GradleBuild.java

@@ -152,6 +152,7 @@ public class GradleBuild {
 				.replace("{version}", getBootVersion())
 				.replace("{dependency-management-plugin-version}", getDependencyManagementPluginVersion());
 		FileCopyUtils.copy(scriptContent, new FileWriter(new File(this.projectDir, "build" + this.dsl.getExtension())));
+		new File(this.projectDir, "settings.gradle").createNewFile();
 		FileSystemUtils.copyRecursively(new File("src/test/resources/repository"),
 				new File(this.projectDir, "repository"));
 		GradleRunner gradleRunner = GradleRunner.create().withProjectDir(this.projectDir)