Update minimum PowerShell version to 7.1.7 by bergmeister · Pull Request #1769 · PowerShell/PSScriptAnalyzer

This is to keep up with the recent deprecation of PS 7.0
https://docs.microsoft.com/powershell/scripting/install/powershell-support-lifecycle#powershell-end-of-support-dates

Also make docs more generic

• 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.