Fix access denied when unloading default user registry hive

Force .NET garbage collection before reg.exe unload to release
open RegistryKey handles left by PowerShell registry cmdlets.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

powershell/Remove-BuiltInApps.ps1

@@ -318,6 +318,8 @@ function Set-DefaultUserProfileDefaults {
     finally {
         if ($HiveWasLoaded) {
             Write-Host "Unloading default user registry hive..." -ForegroundColor Cyan
+            [GC]::Collect()
+            [GC]::WaitForPendingFinalizers()
             & reg.exe unload "HKU\DefaultUser" | Out-Null
 
             if ($LASTEXITCODE -ne 0) {