[flutter_appauth] [flutter_appauth_interface] Add analysis_options in flutter_appauth_platform_interface

[lsaudon]

• add usePubspecOverrides in melos
• Update dart and flutter
• add analysis_options in flutter_appauth_platform_interface
• remove unused class and mocktail

[MaikuB]

Apologies for getting to this late. Are you able to explain the reasons behind the changes in this PR? The main one that sticks out to me is the reason behind bumping the minimum Flutter and Dart SDK requirements. The changes in this PR don't seem to provide enough reason to bump the minimum requirements to limit who can use the plugin

[lsaudon]

I put back the old versions.