[Internal] Move to Java 8 and AGP 7

[samtstern]

Version 8.0 of FirebaseUI can/should require Java 8 to allow us to take on newer dependencies. This also partially unblocks a future move to Kotlin.

cc @rosariopfernandes

[samtstern]

Looks like the latest version of AGP is 7.0.0-beta05:
https://maven.google.com/web/index.html?q=com.android.tools.#com.android.tools.build:gradle:7.0.0-beta05

@rosariopfernandes have you heard anything about when 7.0.0 stable will be out?

[thatfiredev]

@samtstern Not really, but I know Jetpack Compose is going stable this month (rc01 is out). I'm guessing AGP7 will be stable alongside Compose and AS Arctic Fox.

[samtstern]

This has been implemented and released in version 8.0.0