af: Bump `auth0_flutter_platform_interface` to beta version

[Widcket]

• [X] All new/changed/fixed functionality is covered by tests (or N/A)
• [X] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR bumps the version of auth0_flutter_platform_interface to v1.2.0-beta.0.