Remove the Beam/Mixer and Gitter providers

[kevinchalet]

Fixes https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers/issues/762 (Beam/Mixer was shut down by MSFT in 2020). Fixes https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers/issues/763.

I'll unlist the packages on NuGet.org once this PR is merged.

@martincostello any preference for the milestone? No longer shipping a package is technically a breaking change (something we generally want to avoid for a patch version), but since the services are no longer operating... 😄

[martincostello]

If they don't even work anymore, then I think SemVer goes out the window 🤣

[kevinchalet]

That was my feeling too 😄

[martincostello]

I'll sync this with the 8.0 branch tomorrow so there's no more previews published to MyGet either.

[kevinchalet]

Thanks! :heart:

[kevinchalet]

Packages deprecated on NuGet.org:

• https://www.nuget.org/packages/AspNet.Security.OAuth.Beam/
• https://www.nuget.org/packages/AspNet.Security.OAuth.Gitter/

[martincostello]

I'll sync this with the 8.0 branch tomorrow so there's no more previews published to MyGet either.

Done.