Upgrade Gradle to 7.0

[suhtai]

For us to support JDK 16 for the compilation of the project, we need to upgrade to Gradle 7 first, as I found out while adding multiple runs for different JDKs in #2392

[felixdivo]

The README still reads:

JDK 16 doesn't currently work but will do once we upgrade to Gradle 7.0

Was it just not updated? It links here.

[nomisRev]

You're correct. That is out of date in the README.