Upgrade to .NET 7

[xingsy97]

Summary of the changes

• Add .Net 7 support for some packages

Preparation for Client Invocation

[JialinXin]

Is this 5.0 SDK used by Tests projects? Shall we upgrade tests to net7.0 as well so we don't need to install this?

[xingsy97]

@JialinXin

• I encountered this error when executing build.cmd

 Failed to negotiate protocol, waiting for response timed out after 90 seconds. This may occur due to machine slowness, please set environment variable VSTEST_CONNECTION_TIMEOUT to increase timeout.
  Results File: D:\a\azure-signalr\azure-signalr\artifacts\logs\UnitTests-net7.0-t000.trx

• This error can be re-produced both in github and my local environment.
• It's caused by dotnet test/vstest rather than our own UT. Re-run cannot solve it.
• Not found any failed unit test in log. And UT works well under VS in my local environment.
• Upgrading only SDK itself to .NET 7 won't cause this problem. This error appeared after upgrading UT to .NET 7.
• According to the error message, I set the the environment variable VSTEST_CONNECTION_TIMEOUT to 10000 (any large number) by executing $env:VSTEST_CONNECTION_TIMEOUT=10000 in powershell terminal and execute ./build.cmd. Then the test stuck here

  [xUnit.net 00:00:11.36]     Microsoft.Azure.SignalR.Common.Tests.Auth.AzureActiveDirectoryTests.TestAuthenticateAsync [SKIP]
  [xUnit.net 00:00:11.36]     Microsoft.Azure.SignalR.Common.Tests.Auth.AzureActiveDirectoryTests.TestGetAzureAdTokenAndAuthenticate [SKIP]
  [xUnit.net 00:00:11.36]     Microsoft.Azure.SignalR.Common.Tests.Auth.AzureActiveDirectoryTests.TestAcquireAccessToken [SKIP]
    Skipped Microsoft.Azure.SignalR.Common.Tests.Auth.AzureActiveDirectoryTests.TestAuthenticateAsync [1 ms]
    Skipped Microsoft.Azure.SignalR.Common.Tests.Auth.AzureActiveDirectoryTests.TestGetAzureAdTokenAndAuthenticate [1 ms]
    Skipped Microsoft.Azure.SignalR.Common.Tests.Auth.AzureActiveDirectoryTests.TestAcquireAccessToken [1 ms]
  [xUnit.net 00:00:20.43]     Microsoft.Azure.SignalR.Tests.TestEndpointServiceConnectionContainerTests.TestStatusPingChangesEndpointStatus [SKIP]
    Skipped Microsoft.Azure.SignalR.Tests.TestEndpointServiceConnectionContainerTests.TestStatusPingChangesEndpointStatus [1 ms]

It seems the error is not caused by insufficient VSTEST_CONNECTION_TIMEOUT. Maybe the vstest connection was already somehow broken.

• Searched out some information but none of them helps

1. Unable to run test. Test run getting aborted microsoft/vstest#3127
   Its author said the problem was solved by 'newer versions of the Test Platform'.

2. https://stackoverflow.com/questions/61605460/tests-fail-to-run-after-upgrading-net-core-from-2-1-to-3-1
   It said adding package Microsoft.NET.Test.Sdk. Most of our UT projects already have this package

3. Failed to negotiate protocol. Wait for response timeout microsoft/vstest#1607 (comment)
   It said a solution for netcoreapp2.0. I don't know how to use it.

4. https://solveforum.com/forums/threads/solved-vstest-console-process-failed-to-connect-to-testhost-process-after-90-seconds.316059/ and https://developercommunity.visualstudio.com/t/frequent-test-failure-errorrunmessage-vstestconsol/571224
   nothing helpful