(OFBIZ-12192)

Changes script to use the proper Gradle wrapper version. Changes the
download url from Bintray to Gradle GitHub repository and removes the
backup link. Adjusts the sha checksums accordingly.

The Gradle download link pulls a gradle-wrapper.properties file which
references services.gradle.org/distributions/gradle-6.5-bin.zip instead
of a 6.5.1 distribution. This was left this way for consistency.