Remove WiX v3 infrastructure and migrate exclusively to WiX v5 (#41975)

## Summary:

This pull request refactors the installer build pipeline to simplify and
modernize the process, focusing exclusively on the WiX 5 (VNext)
installer and removing legacy WiX 3 support. It eliminates the use of
the `installerSuffix` parameter and related logic, removes the legacy
installer build steps and scripts, and updates documentation to reflect
the new architecture. The changes streamline the pipeline, reduce
complexity, and ensure only the latest installer is built and signed.

Pipeline and build system simplification:

* Removed the `installerSuffix` parameter and all related logic from
pipeline templates and YAML files, including file naming, build steps,
and hash calculation scripts.
* Removed legacy WiX 3 installer build steps and the associated script
`installWiX.ps1`, focusing exclusively on WiX 5 (VNext) installer
builds.

Installer signing and build process updates:

* Updated `.pipelines/ESRPSigning_installer.json` to remove signing
configuration for the legacy `PowerToysSetupCustomActions.dll`, ensuring
only the VNext DLL is signed.

Documentation updates:

* Updated `doc/devdocs/core/installer.md` to remove references to WiX 3,
clarify the installer architecture as WiX 5 only, and describe the new
build process.

## CheckList:
- [ ] Should Build successfully and produce installer for both per user
and per machine
- [ ] Should install without problem

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: vanzue <69313318+vanzue@users.noreply.github.com>
Co-authored-by: Kai Tao (from Dev Box) <kaitao@microsoft.com>

tools/build/build-installer.ps1

@@ -21,12 +21,9 @@ Specifies the build configuration (e.g., 'Debug', 'Release'). Default is 'Releas
 .PARAMETER PerUser
 Specifies whether to build a per-user installer (true) or machine-wide installer (false). Default is true (per-user).
 
-.PARAMETER InstallerSuffix
-Specifies the suffix for the installer naming (e.g., 'wix5', 'vnext'). Default is 'wix5'.
-
 .EXAMPLE
 .\build-installer.ps1
-Runs the installer build pipeline for x64 Release with default suffix (wix5).
+Runs the installer build pipeline for x64 Release.
 
 .EXAMPLE
 .\build-installer.ps1 -Platform x64 -Configuration Release
@@ -36,10 +33,6 @@ Runs the pipeline for x64 Release.
 .\build-installer.ps1 -Platform x64 -Configuration Release -PerUser false
 Runs the pipeline for x64 Release with machine-wide installer.
 
-.EXAMPLE
-.\build-installer.ps1 -Platform x64 -Configuration Release -InstallerSuffix vnext
-Runs the pipeline for x64 Release with 'vnext' suffix.
-
 .NOTES
 - Make sure to run this script from a Developer PowerShell (e.g., VS2022 Developer PowerShell).
 - Generated MSIX files will be signed using cert-sign-package.ps1.
@@ -54,8 +47,7 @@ Runs the pipeline for x64 Release with 'vnext' suffix.
 param (
     [string]$Platform = '',
     [string]$Configuration = 'Release',
-    [string]$PerUser = 'true',
-    [string]$InstallerSuffix = 'wix5'
+    [string]$PerUser = 'true'
 )
 
 # Ensure helpers are available
@@ -97,7 +89,7 @@ if (-not $repoRoot -or -not (Test-Path (Join-Path $repoRoot "PowerToys.sln"))) {
 }
 
 Write-Host "PowerToys repository root detected: $repoRoot"
-# WiX v5 projects use WixToolset.Sdk via NuGet/MSBuild; a separate WiX 3 installation is not required here.
+# WiX v5 projects use WixToolset.Sdk via NuGet/MSBuild; no separate WiX installation is required.
 Write-Host ("[PIPELINE] Start | Platform={0} Configuration={1} PerUser={2}" -f $Platform, $Configuration, $PerUser)
 Write-Host ''
 
@@ -151,8 +143,8 @@ try {
 
 RunMSBuild 'installer\PowerToysSetup.sln' "$commonArgs /t:restore /p:RestorePackagesConfig=true" $Platform $Configuration
 
-RunMSBuild 'installer\PowerToysSetup.sln' "$commonArgs /m /t:PowerToysInstallerVNext /p:PerUser=$PerUser /p:InstallerSuffix=$InstallerSuffix" $Platform $Configuration
+RunMSBuild 'installer\PowerToysSetup.sln' "$commonArgs /m /t:PowerToysInstallerVNext /p:PerUser=$PerUser" $Platform $Configuration
 
-RunMSBuild 'installer\PowerToysSetup.sln' "$commonArgs /m /t:PowerToysBootstrapperVNext /p:PerUser=$PerUser /p:InstallerSuffix=$InstallerSuffix" $Platform $Configuration
+RunMSBuild 'installer\PowerToysSetup.sln' "$commonArgs /m /t:PowerToysBootstrapperVNext /p:PerUser=$PerUser" $Platform $Configuration
 
 Write-Host '[PIPELINE] Completed'