Upgrade to Gradle 6.1

Closes gh-19772

gradle/wrapper/gradle-wrapper.properties

 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.0.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.1-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

@@ -17,7 +17,6 @@
 package org.springframework.boot.gradle.junit;
 
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.List;
 import java.util.stream.Stream;
 
@@ -44,10 +43,10 @@ public final class GradleCompatibilityExtension implements TestTemplateInvocatio
 
 	static {
 		if (JavaVersion.current().isCompatibleWith(JavaVersion.VERSION_13)) {
-			GRADLE_VERSIONS = Collections.singletonList("default");
+			GRADLE_VERSIONS = Arrays.asList("6.0.1", "default");
 		}
 		else {
-			GRADLE_VERSIONS = Arrays.asList("5.6.4", "default");
+			GRADLE_VERSIONS = Arrays.asList("5.6.4", "6.0.1", "default");
 		}
 	}