Make it possible to build ScriptAnalyzer with PowerShell7

[bergmeister]

PR Summary

Because PS7 implements netstandard2.0 we do not need to change anything apart from redirecting 7 to 6.
In the future we could hypothetically use netstandard2.1 features and only then we would need a PS7 specific build.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[JamesWTruher]

looks good