fetchSignInMethodsForEmail: UniversalEmailSignIn and co deprecation

[lesnitsky]

On September 15, 2023 email enumeration protection was enabled by default for all new Firebase projects created after that date to boost security. When this setting is enabled, it limits the ability to check for what existing sign in methods exist for an email address through the fetchSignInMethodsForEmail.

If you do need access to fetchSignInMethodsForEmail, the only workaround is to disable email enumeration protection on your Firebase project's underlying Cloud project. Disabling email enumeration protection will lower the security of your project.

We will be deprecating:

• UniversalEmailSignInScreen
• UniversalEmailSignInProvider
• UniversalEmailSignInController
• DifferentSignInMethodsFound auth state
• AuthProvider#findProvidersForEmail
• AuthListener#onBeforeProvidersForEmailFetch
• AuthListener#onDifferentProvidersFound