Update Gradle to latest version

[ErikSchierboom]

When I open an exercise in IntelliJ, I get a warning message that some Gradle features are no longer supported. It would be nice to not have this warning by upgrading the Gradle version.

[JonForest]

FWIW, we also get this in the command line when running the ./gradlew test command

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.

[incendium]

This can be closed as #505 has resolved this issue.

[SleeplessByte]

Thanks @incendium for the ping.