Fix warning `SYSLIB0050`

[AndreasReitberger]

warning SYSLIB0050: "FormatterAssemblyStyle" ist veraltet: "Formatter-based serialization is obsolete and should not be used." (https://aka.ms/dotnet-warnings/SYSLIB0050)

Coming from

    [JsonSerializable(typeof(FirebaseHandler))]
    [JsonSerializable(typeof(CurrentUserChangedEventArgs))]
    [JsonSerializable(typeof(UserDataChangedEventArgs))]
    [JsonSourceGenerationOptions(WriteIndented = true)]
    public partial class FirebaseSourceGenerationContext : CoreSourceGenerationContext
    { }