Recommend Gradle version to users

[ybyzek]

Tutorials may need a warning to users on which Gradle version to use (or be explicit with --gradle-version 7.4.2).

Otherwise:

FAILURE: Build failed with an exception.

* Where:
Build file '...build.gradle' line: 22

* What went wrong:
...
> Could not get unknown property 'mainClass' for object of type org.gradle.api.plugins.internal.DefaultJavaApplication.

or

A problem was found with the configuration of task ':shadowJar' (type 'ShadowJar').
  - In plugin 'com.github.jengelman.gradle.plugins.shadow.ShadowBasePlugin' type 'com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar' property 'mainClassName' doesn't have a configured value.

Internal Slack thread

cc: Bill Bejeck (@bbejeck)