fix(auth)!: remove non-functional MFA recovery codes

[demolaf]

⚠️ Breaking Changes

Removes the MFA recovery-codes API surface. Pre-GA (10.0.0-beta05), so no deprecation cycle.

• AuthUIStringProvider — recoveryCodesSavedAction, mfaStepShowRecoveryCodesTitle and mfaStepShowRecoveryCodesHelper are gone from the public interface. Anyone with a custom AuthUIStringProvider implementation will fail to compile ('recoveryCodesSavedAction' is not a member of supertype) until those overrides are deleted.
• MfaConfiguration.enableRecoveryCodes — constructor parameter removed. It previously defaulted to true, so callers who never set it are unaffected at the source level; callers who passed it explicitly must drop the argument.
• MfaEnrollmentStep.ShowRecoveryCodes — enum case removed. Any exhaustive when over MfaEnrollmentStep in a custom enrollment slot must drop that branch.
• MfaEnrollmentContentState.recoveryCodes / .onCodesSavedClick — removed.
• String resources — fui_recovery_codes_saved_action, fui_mfa_step_show_recovery_codes_title and fui_mfa_step_show_recovery_codes_helper removed across all 85 locales.

Behaviour change: with recovery codes enabled (the previous default), onComplete fired when the user confirmed they had saved their codes. It now fires as soon as the second factor is verified — one interaction earlier.

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Maintainer note: Fixes internal CPRN-379

[gemini-code-assist]

Code Review

This pull request removes the recovery (backup) codes feature from the Multi-Factor Authentication (MFA) enrollment flow across FirebaseUI Auth, as Firebase Authentication lacks a backup-code primitive to verify them at sign-in. The changes include removing the enableRecoveryCodes configuration option, deleting the ShowRecoveryCodes step and its associated UI components, removing localized string resources for recovery codes across all supported languages, and updating the documentation and tests accordingly. I have no feedback to provide as there are no review comments to assess.