Migrate to google_sign_in 7.x

[timobaehr]

The API of google_sign_in 6.x and earlier was designed for the Google Sign-In SDK, which has been deprecated on both Android and Web, and replaced with new SDKs that have significantly different structures. As a result, the google_sign_in API surface has changed significantly.
https://github.com/flutter/packages/blob/main/packages/google_sign_in/google_sign_in/MIGRATION.md

Steps until this issue is resolved

Override the dependency google_sign_in until this package supports version 7.

dependency_overrides:
  google_sign_in: ^6.3.0

Subtasks

• Use google_sign_in 7.1
• Check if it is required to use clientId or serverClientId
• The app should not always ask which account to use at app start, implement silent login
• The library should work on web
• Check if we really need to show the login button at app start on web, very bad UX