UseSignalR and UseConnections are marked obsolete

## UseSignalR and UseConnections are marked obsolete

The methods `UseConnections` and `UseSignalR`, and the classes `ConnectionsRouteBuilder` and `HubRouteBuilder` are being marked obsolete in 3.0.

### Version introduced

3.0

### Old behavior

SignalR hub routing was configured using `UseSignalR`, `UseConnections`.

### New behavior

The old way of configuring routing has been obsoleted and replaced with Endpoint routing.

### Reason for change

We're moving middleware to the new EndpointRouting system so we're obsoleting the old way of adding middleware.

### Recommended action

Replace `UseSignalR` with `UseEndpoints`,

Old code:
```c#
app.UseSignalR(routes =>
{
    routes.MapHub<SomeHub>("/path");
});
```
New code:
```c#
app.UseEndpoints(endpoints =>
{
    endpoints.MapHub<SomeHub>("/path");
});
```

### Category

ASP.NET Core

### Affected APIs

[Microsoft.AspNetCore.Builder.IApplicationBuilder.UseSignalR](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.builder.signalrappbuilderextensions.usesignalr?view=aspnetcore-2.2#Microsoft_AspNetCore_Builder_SignalRAppBuilderExtensions_UseSignalR_Microsoft_AspNetCore_Builder_IApplicationBuilder_System_Action_Microsoft_AspNetCore_SignalR_HubRouteBuilder__)
[Microsoft.AspNetCore.Builder.IApplicationBuilder.UseConnections](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.builder.connectionsappbuilderextensions.useconnections?view=aspnetcore-2.2#Microsoft_AspNetCore_Builder_ConnectionsAppBuilderExtensions_UseConnections_Microsoft_AspNetCore_Builder_IApplicationBuilder_System_Action_Microsoft_AspNetCore_Http_Connections_ConnectionsRouteBuilder__)
[Microsoft.AspNetCore.Http.Connections.ConnectionsRouteBuilder](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.connections.connectionsroutebuilder?view=aspnetcore-2.2)
[Microsoft.AspNetCore.SignalR.HubRouteBuilder](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.signalr.hubroutebuilder?view=aspnetcore-2.2)



---
#### Issue metadata

* Issue type: breaking-change

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UseSignalR and UseConnections are marked obsolete #14928