Review the list of obsolete types and see whether we can remove any of these in .NET 6

[mkArtakMSFT]

For .NET 6, we'd like to evaluate the APIs marked as obsolete and remove them if necessary. There's ~130 APIs marked as obsolete that are split up based on project area in the table below. The goal is to have these APIs removed by preview1 so we have time to get feedback on our decision early on in the release.

What to do

• If it should be removed
  • Remove it
  • Update any samples/tests that use the obsolete API.
• If it should not be removed:
  • Add an inline comment explaining why and provide a timeline for removal if possible
  • Ensure that the Obsolete description provides information about alternative APIs or guidelines on what the user should do otherwise.
  • Provide an aka.ms link to the deprecation announcement.

Who is doing it

Area	Assigned	Done?
Azure AD	@HaoK, @JunTaoLuo, @javiercn	#27955
Hosting	@Tratcher, @halter73	#27529 (comment)
Http	@Tratcher, @jkotalik , @halter73	#28890
Middleware	@Tratcher, @HaoK, @halter73	#28893
MVC	@pranavkm, @javiercn, @captainsafia	#28337
Razor	@ajaybhargavb, @captainsafia	#28706
Servers	@jkotalik, @halter73	#29275
SignalR	@BrennanConroy	#28100

Things to keep in mind
@halter73 and @captainsafia took a look at all the APIs and made the following list of things to consider when evaluating whether or not an Obsoleted API should be removed.

• Outside of redundancy, is there another harm to having the API around? For example, it is not performant or not supported by other parts of the ecosystem.
• What's the maintenance cost of keeping the API around? This includes things like the number of issues reported by users, whether there's a lot of flaky tests associated with the API, etc.
• Does the Obsoleted API no longer work?
• How common is the API that is being obsoleted? Is it likely a new user will inadvertently use it and run into issues?
• Is it actually a shipped API?
• When was the API obsoleted? If it was obsoleted in .NET 5, keep it around to make upgrading between LTS releases, from 3.1 to 6, easier for devs.
• Is this API commonly used by other packages?

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[Pilchie]

Also to keep in mind - if the API was first marked Obsolete in .NET 5.0, we might consider keeping it for .NET 6 to make it easier for people to upgrade from LTS to LTS (.NET Core 3.1 -> .NET 6.0).

[javiercn]

We should consider adding aka.ms links to the obsolete message that point to a landing doc (the announcement?) and that suggest alternatives.