Reverts "Upgrade template Gradle, App AGP, Module AGP, and Kotlin versions, and tests (#150969)" (#151147)

Reverts: flutter/flutter#150969
Initiated by: goderbauer
Reason for reverting: Failing test in https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8743574743030691569/+/u/run_android_obfuscate_test/stdout
Original PR Author: gmackall

Reviewed By: {christopherfujino, reidbaker}

This change reverts the following previous change:
After the land of https://github.com/flutter/engine/pull/53592, there is some log spam:
```
e: /Users/mackall/.gradle/caches/transforms-3/c1e137371ec1afe9bc9bd7b05823752d/transformed/fragment-1.7.1/jars/classes.jar!/META-INF/fragment_release.kotlin_module: Module was compiled with an incompatible version of Kotlin. The binary version of its metadata is 1.8.0, expected version is 1.6.0.
e: /Users/mackall/.gradle/caches/transforms-3/d86c7cb1c556fe1655fa56db671c649c/transformed/jetified-activity-1.8.1/jars/classes.jar!/META-INF/activity_release.kotlin_module: Module was compiled with an incompatible version of Kotlin. The binary version of its metadata is 1.8.0, expected version is 1.6.0.
...
```

I think this is harmless, but still annoying. Upgrading the AGP version fixes it. To be honest, I don't know why (I expected the Kotlin version would do it). But after https://github.com/flutter/flutter/pull/146307, our tests have been running on AGP/Gradle 8.1/8.3 for a while, so it makes sense to upgrade anyways.

In a follow up PR:
1. Also upgrade the tests that were left behind in https://github.com/flutter/flutter/pull/146307, as I think removal of discontinued plugins paved the way here.