[Auth] Match Firebase 10 implementation in WKNavigationDelegate conformance

[ncooke3]

While testing #13634, found one behavior regression that led to reCATPCHA not working on macCatalyst.

Steps to repro:

1. Build Auth sample for macCatalyst (requires some disabling of code to get build green, and enabling Catalyst support)
2. Phone number flow, add phone #, verify
3. A blank screen will be presented

With this PR, the reCAPTCHA flow will be presented instead of a blank screen.

In v10, allow was always returned:

firebase-ios-sdk/FirebaseAuth/Sources/Utilities/FIRAuthWebViewController.m

Line 87 in eca84fd

decisionHandler(WKNavigationActionPolicyAllow);

[google-oss-bot]

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[paulb777]

LGTM

firebase-ios-sdk/FirebaseAuth/Sources/Utilities/FIRAuthWebViewController.m

Line 87 in eca84fd

decisionHandler(WKNavigationActionPolicyAllow);