[release/10.0.4xx] Backport #7761: update WiX toolset to v6 - #7765

Backports the intent of #7761 onto release/10.0.4xx: move installer/tooling integration to WiX v6 and align branch-local SDK/package wiring.
This applies the same effective updates where files still exist on this branch, with conservative conflict resolution for branch-specific deletions.

• Arcade WiX version + installer task updates

  • src/arcade/Directory.Packages.props: MicrosoftWixToolsetSdkVersion → 6.0.3-dotnet.4
  • src/arcade/src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props: same version bump
  • src/arcade/src/Microsoft.DotNet.Build.Tasks.Installers/build/wix5/wix.targets
    • WixExtensionsDir: wixext5 → wixext6
    • harvest args include -sw5149
  • src/arcade/src/Microsoft.DotNet.Build.Tasks.Installers/build/wix5/bundle/bundle.wxs
    • RelatedBundle attribute updated from Id to Code (WiX v6 schema)

• SDK-side alignment

  • src/sdk/global.json: Microsoft.WixToolset.Sdk → 6.0.3-dotnet.4
  • src/sdk/src/Layout/pkg/windows/Directory.Build.props: set HarvestDirectorySuppressSpecificWarnings=5149

• Branch adaptation during backport

  • The following paths from source PR are deleted on release/10.0.4xx and were intentionally kept deleted:
    • src/aspnetcore/global.json
    • src/runtime/eng/Versions.props
    • src/windowsdesktop/eng/Versions.props
    • src/windowsdesktop/global.json
    • src/windowsdesktop/src/windowsdesktop/src/bundle/bundle.wxs

<!-- wix.targets -->
<WixExtensionsDir>wixext6</WixExtensionsDir>
...
AdditionalOptions="$(AdditionalHeatArgs) -sw5149"