Update target and compileSdkVersion to 33

[louisjennings]

In order to update the app on the Google Play Store, it is necessary to target SDK version 33 or 34. This PR updates the target SDK version and the min SDK version to resolve the issues raised in #71.

Changes:

• Update target and compileSdkVersion to 33
• Update minSdkVersion to 21
• Remove unnecessary buildToolsVersion directive