Add net6.0 TFM to ExternalAccess.Razor

[jjonescz]

Fixes MissingMethodExceptions in Razor tests after updating Roslyn to a version after #66655. These occur because of mismatch in IsExternalInit attribute (credit for discovering the problem goes to @davidwengier).

Details (reference graphs):

Previously, this worked:

• Microsoft.AspNetCore.Razor.LanguageServer.Test net7.0
  • MS.CA.ExternalAccess.Razor netstandard2.0
    • MS.CA.Workspaces netstandard2.0
      • has its own IsExternalInit attribute embedded ✅
  • MS.CA.Workspaces netcoreapp3.1
    • has its own IsExternalInit attribute embedded ✅

After #66655, it broke:

• Microsoft.AspNetCore.Razor.LanguageServer.Test net7.0
  • MS.CA.ExternalAccess.Razor netstandard2.0
    • MS.CA.Workspaces netstandard2.0
      • has its own IsExternalInit attribute embedded ❌
  • MS.CA.Workspaces net6.0 👈 changed
    • uses IsExternalInit attribute from System.Runtime ❌

After this PR, it works again:

• Microsoft.AspNetCore.Razor.LanguageServer.Test net7.0
  • MS.CA.ExternalAccess.Razor net6.0 👈 changed
    • MS.CA.Workspaces net6.0
      • uses IsExternalInit attribute from System.Runtime ✅
  • MS.CA.Workspaces net6.0
    • uses IsExternalInit attribute from System.Runtime ✅

(Alternative would be making Microsoft.AspNetCore.Razor.LanguageServer.Test target net472. I tried that but got AppDomainUnloaded exceptions in CI.)