Improve Pester bootstrap logic for CI (#1853)

Author: bergmeister

README.md

@@ -163,8 +163,7 @@ built-in rules.
 
 Pester-based ScriptAnalyzer Tests are located in `path/to/PSScriptAnalyzer/Tests` folder.
 
-- Ensure [Pester 4.3.1](https://www.powershellgallery.com/packages/Pester/4.3.1) or higher is
-  installed
+- Ensure [Pester](https://www.powershellgallery.com/packages/Pester) of at least version 5.3 is installed
 - In the root folder of your local repository, run:
 
 ```powershell

tools/appveyor.psm1

@@ -4,8 +4,8 @@
 $ErrorActionPreference = 'Stop'
 
 function Install-Pester {
-    $requiredPesterVersion = '5.2.2'
-    $pester = Get-Module Pester -ListAvailable | Where-Object { $_.Version -eq $requiredPesterVersion }
+    $requiredPesterVersion = '5.3'
+    $pester = Get-Module Pester -ListAvailable | Where-Object { $_.Version -ge $requiredPesterVersion }
     if ($null -eq $pester) {
         if ($null -eq (Get-Module -ListAvailable PowershellGet)) {
             # WMF 4 image build