Upgrade to latest johnrengelman.shadow dependency to avoid vulnerable log4j version

[henbox]

Example step gradle builds currently use 'com.github.johnrengelman.shadow' version '5.2.0', which will cause dependency log4j-core and log4j-api 2.11 to be downloaded to the build machine.

Upgrade to 7.1.2 so that log4j 2.17.1 is used instead: https://github.com/johnrengelman/shadow/releases

For this change we'll also need to upgrade to Gradle v7. In gradle-wrapper.properties:
distributionUrl=https://services.gradle.org/distributions/gradle-7.5.1-bin.zip