[Java] Upgrade to Gradle 6.4.1 and javadoc-links 5.1.0.

Author: mjpt777

build.gradle

@@ -22,7 +22,7 @@ buildscript {
 plugins {
     id 'java-library'
     id 'idea'
-    id 'io.freefair.javadoc-links' version '5.0.1' apply false
+    id 'io.freefair.javadoc-links' version '5.1.0' apply false
     id 'com.github.johnrengelman.shadow' version '5.2.0' apply false
     id "com.github.ben-manes.versions" version "0.28.0"
 }
@@ -686,6 +686,6 @@ task testReport(type: TestReport) {
 }
 
 wrapper {
-    gradleVersion = '6.4'
+    gradleVersion = '6.4.1'
     distributionType = 'ALL'
 }

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.4-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.4.1-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists