Developers should be guided away from more obsolete APIs in .NET 6

[jeffhandley]

With #39269, we marked many APIs as [Obsolete] in .NET 5. Some follow-up infrastructure work was identified during and after that PR, and some other obsoletions have been proposed. With this story, we will:

1. Complete that follow-up infrastructure work
2. Consider additional APIs that could be marked as [Obsolete] (optionally with custom diagnostic Ids)
3. Document those obsoletions
4. Consider if there are places where a Roslyn Code Fixer should be provided for obsolete APIs

.NET 6 Work Items

• Consider marking net5.0 obsoleted APIs obsolete in downlevel targets for OOB packages Consider marking net5.0 obsoleted APIs obsolete in downlevel targets for OOB packages #43484
• For .NET 5 Obsoletions - use partial ref source classes instead of ifdefs For .NET 5 Obsoletions - use partial ref source classes instead of ifdefs #39687
• ImageAnimator.StopAnimate (on Unix) could throw PNSE ImageAnimator.StopAnimate (on Unix) could throw PNSE #39405
• Obsolete warnings on clean build Obsolete warnings on clean build #39976
• Add SYSLIB0013 into obsoletion list Add SYSLIB0013 into obsoletion list #44203 (this just needs documentation)
• API proposal: obsolete DisablePrivateReflectionAttribute API proposal: obsolete DisablePrivateReflectionAttribute #43247
• Obsolete SuppressILDasmAttribute Obsolete SuppressIldasmAttribute #40622 (PR Obsolete SuppressIldasmAttribute and remove ildasm.exe support for it #50951)
• WebRequest/ServicePoint/WebClient Obsoletion Follow-Up WebRequest/ServicePoint/WebClient Obsoletion Follow-Up #41759
• API proposal: Obsolete RNGCryptoServiceProvider API proposal: Obsolete RNGCryptoServiceProvider #40169 (PR: Mark RNGCryptoServiceProvider as Obsolete #52373)
• Consider marking HandleProcessCorruptedStateExceptionsAttribute as obsolete/hidden Consider marking HandleProcessCorruptedStateExceptionsAttribute as obsolete/hidden #31157
• Obsolete the Rijndael and RijndaelManaged classes Obsolete the Rijndael and RijndaelManaged classes #46930 (PR: Obsolete the Rijndael and RijndaelManaged classes #52366)
• Obsolete unnecessary cryptographic derived types Obsolete unnecessary cryptographic derived types #46934 (PR: Unnecessary cryptographic derived types obsoletions #52303)
• 32 bit memory getters on Process should not wrap 32 bit memory getters on Process should not wrap #46901
• Developers are directed to helpful documentation for existing obsoletions Developers are directed to helpful documentation for existing obsoletions #47255

Moved to Future

• Mark StringDictionary as obsolete Mark StringDictionary as obsolete #889
• Proposed code fixer for obsoleted CAS APIs Proposed code fixer for obsoleted CAS APIs #39235
• Assembly.CodeBase obsoletion is incomplete Assembly.CodeBase obsoletion is incomplete #42857
• [Meta] List of APIs considered for "Better Obsoletion" [Meta] List of APIs considered for "Better Obsoletion" #33360

Definition of Done

Each obsoletion needs to have the following Definition of Done applied.

• Pre-announce the upcoming breaking change to partner teams using our breaking change DL at least 2 weeks before merge
• Use the Obsoletions.cs file to define the message and diagnostic ID
• Add the obsoletion to the list of diagnostics
• Announce the breaking change to our partner teams using our breaking change DL upon merge
• Create the breaking change document issue
• Register the aka.ms link for SYSLIBxxxx obsoletions (see @jeffhandley)

[jeffhandley]

/cc @terrajobst @stephentoub @jkotas @GrabYourPitchforks in case any of you has other items you'd like to add to this list.