Recommend Gradle version to users

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](https://confluent.slack.com/archives/CHMFCJRCM/p1654512038548539)

cc: @bbejeck