Add new method to update Auth User's email

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Add FirebaseUser.SendEmailVerificationBeforeUpdatingEmailAsync, which is the new method to update user emails,
and deprecate UpdateEmailAsync, which no longer works when Email Enumeration Protection is turned on.

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Testing

Describe how you've tested these changes.

Building and testing locally, plus
https://github.com/firebase/firebase-unity-sdk/actions/runs/8103734621

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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