Developers should be guided away from more obsolete APIs in .NET 7

[jeffhandley]

Continuing on the momentum from #39269, #46767, and #33360, .NET 7 should guide developers away from more APIs that are considered obsolete but haven't yet been annotated as such.

Planned for .NET 7

• Assembly.CodeBase obsoletion is incomplete #42857
• Obsolete thumbtacked AssemblyName properties  #59061
• Obsolete Regex.CompileToAssembly #50535 (PR Obsolete Regex.CompileToAssembly #59734)

Backlog (in roughly priority order)

• Mark StringDictionary as obsolete #889
  • This requires proposing replacement APIs for all places StringDictionary is used in public APIs (including aspnetcore)
• Feature proposal: remove implicit construction of DateTimeOffset from DateTime #32954
• SecureString obsoletions and shrouded buffer proposal designs#147
• Proposed code fixer for obsoleted CAS APIs #39235

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
• Create the breaking change document issue
• Announce the breaking change to our partner teams using our breaking change DL upon merge
• Register the aka.ms link for SYSLIBxxxx obsoletions (see @jeffhandley)

[AraHaan]

I think obsolete types should face the same fate as BinaryFormater and be deleted in .NET 8.