Gradle and major dependency updates

[TacoTheDank]

I know that much of this probably isn't necessary, since you're migrating the entire project to Kotlin, but it was fun to practice. Also I don't know what libraries you intend to use in the Kotlin app (there's a nifty library called RxKotlin). Maybe you could release a final 5.x version with the last changes before 6.

1st commit

• Delete currently unused files

2nd commit

• Update gradle wrapper to 6.7 using git bash

3rd commit

• Label dependencies
• Make both modules compatible with Java 1.8 (Kotlin JVM targeting 1.8)
• Use Kotlin JDK8
• Update Kotlin 1.3.50 -> 1.4.10 (changelog)
• Update Android Studio 3.5.2 -> 4.1.1 (changelog)

4th commit

• Comment out unused firebase crash library
• Update billing library 2.0.3 -> 2.2.0 (changelog)
• Update GMS plugin 3.1.2 -> 4.3.4 (you might have to update some stuff in your google-services.json) (changelog)

5th commit

• Update AndroidX AppCompat to 1.2.0 (changelog)
• Update AndroidX Core 1.1.0 -> 1.3.2 (changelog)
• Remove unused legacy support library
• Update AndroidX Lifecycle 2.1.0 -> 2.2.0 (and fixing deprecations) (changelog)
• Update AndroidX Navigation 2.1.0 -> 2.3.1 (changelog)
• Update AndroidX Preference 1.1.0 -> 1.1.1 (changelog)
• Update AndroidX RecyclerView to 1.1.0 (changelog)
• Update Google Material 1.0.0 -> 1.2.1 (changelog)
• Update Retrofit 2.3.0 -> 2.6.4 (changelog)
• Update Gson 2.8.0 -> 2.8.6 (changelog)
• Update RxJava 1.3.0 -> 1.3.8 (changelog)
• Update Stetho 1.5.0 -> 1.5.1 (changelog)

6th commit

• Migrate RxJava to RxJava2 (changelog)

7th commit

• Migrate the last of App to AppCompat

[physphil]

Thanks for taking care of this 🙂You're right, I am working on a Kotlin rewrite, but like most things it's taking longer than expected. I'll take a look at this PR soon.