8240539: Upgrade gradle to version 6.3

Reviewed-by: jvos, arapte

Author: kevinrushforth

build.properties

@@ -84,7 +84,7 @@ jfx.build.jdk.buildnum.min=28
 # gradle/legal/gradle.md.
 # The jfx.gradle.version.min property defines the minimum version of gradle
 # that is supported. It must be <= jfx.gradle.version.
-jfx.gradle.version=6.0
+jfx.gradle.version=6.3
 jfx.gradle.version.min=5.3
 
 # Toolchains

gradle/legal/gradle.md

@@ -1,4 +1,4 @@
-## Gradle v6.0
+## Gradle v6.3
 
 ### Apache 2.0 License
 ```

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.0-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists