Upgrade Gradle to 7.6 and lower minSdkVersion requirement to 21 (close #587 and #585)

[matus-tomlein]

• Downgrade minSdkVersion to 21 and compileSdkVersion to 33 (#585)
  • Add desugaring for Java 8 features on older SDKs
• Upgrade Gradle to 7.6.1 (#587)
  • Update buildToolsVersion to 30.0.3
  • Update Kotlin version to 1.8
• Update publishing
  • Remove com.github.dcendents:android-maven-gradle-plugin and com.vanniktech:gradle-maven-publish-plugin
  • Make use of maven-publish provided with AGP
• Rename namespace of Java demo app to distinguish from the Kotlin demo
• Upgrade tracker, demo app and test dependencies