refactor: use "firebase" instead of "FirebaseExtended" as organisation in all links for this repository

[nilsreichardt]

Description

All links of the repository in the codebase are using "FirebaseExtended" instead of "firebase" as GitHub organization. The reason for is this that this repository moved to the Firebase organization.

The old links are still working, because GitHub redirects to the new organization. But this has some disadvantages:

• Confusing for new developers
• Can easily break a new repository with name "FlutterFire" will be created inside the FirebaseExtended organization (e. g. forking this repository).

Therefore, it's a good idea to just update the links.

Related Issues

Closes #8790

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.