refactor: remove IsWindows PS 5.1 compatibility shim

PSModule targets PowerShell LTS (7.6+) where $IsWindows is a built-in
automatic variable. The Desktop edition shim and its PSScriptAnalyzer
suppressions are no longer needed.

Author: MariusStorhaug

src/helpers/Build/Build-PSModuleRootModule.ps1

@@ -131,17 +131,6 @@ $MyInvocation.MyCommand.ScriptBlock.Module.OnRemove = {
         #endregion - Analyze source files
 
         #region - Module header
-        Add-Content -Path $rootModuleFile -Force -Value @'
-[Diagnostics.CodeAnalysis.SuppressMessageAttribute(
-    'PSAvoidAssignmentToAutomaticVariable', 'IsWindows',
-    Justification = 'IsWindows does not exist in PS5.1'
-)]
-[Diagnostics.CodeAnalysis.SuppressMessageAttribute(
-    'PSUseDeclaredVarsMoreThanAssignments', 'IsWindows',
-    Justification = 'IsWindows does not exist in PS5.1'
-)]
-'@
-
         $headerFilePath = Join-Path -Path $ModuleOutputFolder -ChildPath 'header.ps1'
         if (Test-Path -Path $headerFilePath) {
             Get-Content -Path $headerFilePath -Raw | Add-Content -Path $rootModuleFile -Force
@@ -162,10 +151,6 @@ $script:PSModuleInfo | Format-List | Out-String -Stream | ForEach-Object { Write
 $scriptName = $script:PSModuleInfo.Name
 Write-Debug "[$scriptName] - Importing module"
 
-if ($PSEdition -eq 'Desktop') {
-    $IsWindows = $true
-}
-
 '@
         #endregion - Module post-header