Build fail if unity switched to Unity 6 preview

[TNemecek]

Describe the bug
When switching to the Unity 6, build fails in unity with this error:

Launching lib\main.dart on ASUS Z017D in debug mode...
Running Gradle task 'assembleDebug'...
You are applying Flutter's app_plugin_loader Gradle plugin imperatively using the apply script method, which is deprecated and will be removed in a future release. Migrate to applying Gradle plugins with the declarative plugins block: https://flutter.dev/go/flutter-gradle-plugin-apply


FAILURE: Build completed with 2 failures.

1: Task failed with an exception.
-----------
* Where:
Build file 'C:\Projects\UNITY projects\improvio\android\unityLibrary\build.gradle' line: 2

* What went wrong:
A problem occurred evaluating project ':unityLibrary'.
> Could not read script 'C:\Projects\UNITY projects\improvio\android\shared\keepUnitySymbols.gradle' as it does not exist.

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
==============================================================================

2: Task failed with an exception.
-----------
* What went wrong:
A problem occurred configuring project ':unityLibrary'.
> compileSdkVersion is not specified. Please add it to build.gradle

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
==============================================================================

* Get more help at https://help.gradle.org

BUILD FAILED in 1s
Error: Gradle task assembleDebug failed with exit code 1

To Reproduce
Steps to reproduce the behavior:

1. Switch unity version to 6 (preview)
2. Build release flutter from unity
3. Build flutter app
4. See error when build failed

Expected behavior
Build app

Screenshots

Unity (please complete the following information):

• OS: Windows 11
• Unity 6 Preview

Smartphone (please complete the following information):

• Device: Asus
• OS: Android
• Version 8

Additional context
It may be still preview, bud it appears that it will need some changes to fix.

[timbotimbo]

Unity 2023 broke android compatibility, so 6 isn't supported yet either.
#839

[TNemecek]

Jeslti se mohu zeptat, v jakém stavu se tedy nachází fix?
V Unity 6 je práví řada nových funkcionalit, které by se nám výrazně hodili, případně jestli se musíme spokojit zatím s nižší verzí.

EDIT: translation (timbotimbo)

If I may ask, what is the condition of the fix?
There are a number of new features in Unity 6 that we could use, or if we have to settle for a lower version for now.

[timbotimbo]

I added a branch to test an alpha implementation for Unity 6.
Check #967 for instructions.