update theme and build files

[eikaramba]

see

• https://docs.flutter.dev/release/breaking-changes/material-3-migration
• https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply

[Benimautner]

Thanks for the PR. Two things: Is the raise of minsdk required? I'd prefer keeping it at 21, but if the new flutter version depreciates it, we can raise it. Secondly, please add the "-beta" to the version tag.

[eikaramba]

yeah unfortunately there was a problem building the app with the latest sdk/environment. i needed to change a lot as you might see so i cannot recollect exactly what was the problem with the minsdk. but i will check it next time i build the app and change it back should it work somehow now.

[Benimautner]

Hey, FYI, I applied this as a patch to the branch I'm currently working on and spent too much time fighting with kotlin, gradle and flutter :').

https://github.com/go-vikunja/app/tree/view_update

Sorry to merge it in a way that doesn't show it was your contribution btw.