Mark InitialSessionState.ImportPSSnapIn as Obsolete

[daxian-dbw]

PR Summary

Mark InitialSessionState.ImportPSSnapIn as Obsolete.
InitialSessionState.ImportPSSnapIn attempts to load a custom PSSnapin from the registry. Custom PSSnapin has been deprecated and removed from PowerShell Core for a long time.

I think this PR should be included in GA, so as to prevent (hopefully) dependency on it for 7.0 users.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).

[daxian-dbw]

@PoshChan Please retry windows

[PoshChan]

@daxian-dbw, successfully started retry of PowerShell-CI-Windows

[ghost]

🎉v7.0.0-rc.2 has been released which incorporates this pull request.:tada:

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