PR Summary

This pull request includes updates to the build.psm1 script, several nuget.config files, and the tools/findMissingNotices.ps1 script. The changes introduce a new source option, update package source URLs, and fix a regex pattern.

Updates to build.psm1:

• Added 'NuGetOnly' to the ValidateSet for the $Source parameter in the Switch-PSNugetConfig function.
• Updated the Switch-PSNugetConfig function to handle the new 'NuGetOnly' source option.

Updates to nuget.config files:

• Changed the PowerShell package source URL in nuget.config, src/Modules/nuget.config, and test/tools/Modules/nuget.config to remove the preview test version.

Updates to tools/findMissingNotices.ps1:

• Added a check to ensure the key does not already exist in the $existingRegistrationTable before adding a new entry.
• Fixed a regex pattern in the ConvertTo-SemVer function to correctly match the version format.

PR Context

PR Checklist

• PR has a meaningful title
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• 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.
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• Breaking changes
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  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
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  • Documentation needed
    - [ ] Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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  • 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.
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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode
      (which runs in a different PS Host).
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    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
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    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).
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