fix(ui_auth): widen app_links constraint to allow 7.x

[bjrochem72]

Description

firebase_ui_auth currently pins app_links: ^6.4.0. On iOS, the app_links 6.x line still registers through the deprecated UIApplicationDelegate lifecycle events, so every app depending on firebase_ui_auth sees an AppLinksIosPlugin deprecation warning in the launch console. app_links 7.x adopts the UIScene lifecycle and clears it.

This PR widens the constraint to ">=6.4.0 <8.0.0":

• Projects on recent Flutter resolve app_links 7.x and the warning disappears.
• Projects on older Flutter keep resolving 6.x (app_links 7.0.0 requires Flutter 3.38.1+), so no minimum-version bump is needed on firebase_ui_auth's side.

No code changes are required: this package's use of the app_links API is limited to the AppLinks() constructor and uriLinkStream (email_link_auth_provider.dart, email_verification.dart), both unchanged in 7.x — the app_links 7.0.0 changelog explicitly notes it remains backward compatible with v6 setups.

Verified locally with app_links 7.2.1 resolved in the workspace:

• dart analyze packages — no issues across all packages
• firebase_ui_auth unit tests — 69/69 passing
• firebase_ui_oauth unit tests — 7/7 passing

Related Issues

Fixes #672

Checklist

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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 (constraint-only change, no behavioral change; existing tests cover the affected surface).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///) — not applicable, no code changes.
• The analyzer (melos run analyze) does not report any problems on my PR.
• All unit tests pass (melos run test:unit:all doesn't fail).
• 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.

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[gemini-code-assist]

Code Review

This pull request updates the dependency constraint for app_links in packages/firebase_ui_auth/pubspec.yaml from ^6.4.0 to ">=6.4.0 <8.0.0" to expand compatibility. There are no review comments, and I have no feedback to provide.

[bjrochem72]

@googlebot I signed it!