Update android compileSdk and Java version.

[timbotimbo2]

Description

Updated some values in the outdated build.gradle of the plugin.
The example was updated to match it.

• Updated compileSdk to 34.

The plugin is still at compileSdk 29.
34 is currently the minimum for a project to publish on the Play Store, and that is soon to be 35.

Some errors (like #845, #870) are directly caused by this low sdk.

• Updated Java 8 to 11

Unity 2021.3 and 2022.3 both use Java 11.

And i'm also getting console warnings about Java 8 like:

warning: [options] source value 8 is obsolete and will be removed in a future release
warning: [options] target value 8 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.

I can still make a release build with the plugin at Java 11 and my Flutter project at Java 8. So this is (partially?) backwards compatible.

I updated the kotling and AGP version to be somewhat in line with these changes.
These don't seem to matter much as Flutter projects on much newer Kotlin or AGP versions never show any warnings or errors related to them.
Just to be safe, I kept AGP below 8.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore