Gradle support

[stleary]

What problem does this code solve?

• Gradle build support for the unit tests was omitted in the merge. This PR restores it.
• In order to init Gradle from the maven pom.xml file, a newer version of Gradle was needed (6.4).
• Using the new Gradle version caused some JavaDoc errors.
• Some unit tests failed using the new Gradle version. In most cases, the test assumed ordering of JSONObject items.
• Unit tests need the same license as the source code. This was not needed previously since they belonged to different projects.

Risks
Low, no changes to existing lib functionality

Changes to the API?
No

Will this require a new release?
No

Should the documentation be updated?
NO

Does it break the unit tests?
NO

Was any code refactored in this commit?
NO

Review status
APPROVED (by me)
Comment window will be reduced to 1 day since other changes are pending

[johnjaylward]

The gradle build looks like it will compile to java1.7. If we want to maintain 1.6 releases, I think this can't be used for deployment.

[stleary]

Agreed. The next PR will remove/modify selected unit tests so the suite can run under Java 1.6. Deployment to Maven Central can happen after this. I understand there is a substantial sentiment to update the Java version, but one of the primary goals of this project is to avoid breaking applications when new code is released.

[johnjaylward]

I think going forward it may make sense for us to just say something like this in the README:

This library supports Java 8+ (or 9+ or whatever LTS version is+)
If you are running Java 1.6 or Java 1.7, please use {{link to last released build for supported java version}}

I personally haven't used anything but Java8+ in the last 5 years, and before that was using Java 7 for a year and I feel that I was a slow adopter, so I haven't used java1.6 or older in around 6-7 years.

I think if anyone needs to work with code that old, it may be best for them to either fork the project or just use an older release.

We can have that discussion outside of this PR, but with the new way that Oracle is maintaining LTS releases and the support for Modules in 9+, it may be time to make a choice for dropping support for older versions.