Upgrade project's build-time dependencies

[jozuasijsling-ing]

Let's upgrade tooling so that developers with recent Android Studio versions can build the project out of the box.

• Upgraded AGP (Android Gradle Plugin) to 8.4.0
• Embedded Gradle wrapper version 8.7
  _{AGP 8.4.0 needs at least Gradle 8.6, while Android Studio Jellyfish (current latest stable) ships with Gradle 8.4}
• Changed root module's compileSdk version to 34
  _{making it the same as the library modules}
• Disabled Jetifier
  _{as it is not needed according to ./gradlew checkJetifier}
• Removed multiDexEnabled instruction
  _{as it is enabled by default}
• Removed unused module :uiunderstanding
• Removed unused plugin declaration for aspectjtools
• Reworked build.sh _{, so that it…}
  • _{…works even when started from another directory}
  • _{…uses Gradle wrapper for downloading the Gradle binary}
  • _{…relies on AGP for detecting and reporting missing environment variables}