fix(deps): update dependency google_sign_in to v7

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
google_sign_in (source)	dependencies	major	^6.2.1 -> ^7.0.0

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Release Notes

flutter/packages (google_sign_in)

v7.1.1

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• Adds more documentation about the requirement to call initialize exactly
  once.
• Adds information about GoogleSignInException to the migration guide.

v7.1.0

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• Adds the missing export of GoogleSignInExceptionCode, to allow for
  structured error handling as intended by the API.

v7.0.0

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• BREAKING CHANGE: Many APIs have changed or been replaced to reflect the
  current APIs and best practices of the underlying platform SDKs. For full
  details, see the README and migration guide, but notable highlights include:
  • The GoogleSignIn instance is now a singleton.
  • Clients must call and await the new initialize method before calling any
    other methods on the instance.
  • Authentication and authorization are now separate steps.
  • Access tokens and server auth codes are obtained via separate calls.

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