Update Gradle wrapper to v6.5 (support building on JDK 14)

The compile time dependency on Groovy in the 'format' subproject had
to be updated as well. Groovy only works on a Java 14 JVM since
v2.5.10. Any new APIs added between v2.3.10 and v2.5.10 must not be
used. Gradle versions that bundle Groovy versions < v2.5.10 will
otherwise stop working.

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.5-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

subprojects/format/build.gradle

@@ -9,9 +9,11 @@ repositories {
 sourceCompatibility = 1.8
 
 dependencies {
-  implementation 'org.codehaus.groovy:groovy-all:2.3.10' // 2.3.10 ships with Gradle 2.6
+  implementation 'org.codehaus.groovy:groovy-all:2.5.10' // v2.5.10 is the first version which runs on a Java 14 JVM.
+                                                         // At runtime, we must still support v2.3.10 (bundled with
+                                                         // Gradle v2.6) on Java < 14 JVMs.
   implementation 'com.google.guava:guava:28.0-jre'
-  testImplementation('org.spockframework:spock-core:1.1-groovy-2.3') {
+  testImplementation('org.spockframework:spock-core:1.3-groovy-2.5') {
      exclude group: 'org.codehaus.groovy', module: 'groovy-all'
   }