`ElevationRequirement: elevationProhibited` appears to not be respected unless it's at the root level.

Brief description of your issue

• Noticed by @aisgbnok in https://github.com/microsoft/winget-pkgs/pull/155308:

Why does 910cce4 pass validation (comment), but 84484dc fail validation (comment)? The only difference being ElevationRequirement: elevationProhibited in the YAML root (pass) vs individual installer nodes (fail)?

• This is unexpected, because the 1.6.0 schema documentation has ElevationRequirement nested under Installers.

• Also impacted https://github.com/microsoft/winget-pkgs/pull/154823

Steps to reproduce

• Have a manifest for a package which requires ElevationRequirement: elevationProhibited.
• Move that node from the root level to under the Installers level.

Expected behavior

The package manager would install the pacakge in a non-elevated way.

Actual behavior

The package manager attempts an elevated install, which fails.

Environment

https://github.com/microsoft/winget-pkgs/pull/154823#issuecomment-2130019800

Windows Package Manager v1.7.11261
Copyright (c) Microsoft Corporation. All rights reserved.

Windows: Windows.Desktop v10.0.19044.1288
System Architecture: X64
Package: Microsoft.DesktopAppInstaller v1.22.11261.0

Winget Directories
-----------------------------------------------------------------------------------------------------------------------
Logs                               %LOCALAPPDATA%\Packages\Microsoft.DesktopAppInstaller_8wekyb3d8bbwe\LocalState\Diag…
User Settings                      %LOCALAPPDATA%\Packages\Microsoft.DesktopAppInstaller_8wekyb3d8bbwe\LocalState\sett…
Portable Links Directory (User)    %LOCALAPPDATA%\Microsoft\WinGet\Links
Portable Links Directory (Machine) C:\Program Files\WinGet\Links
Portable Package Root (User)       %LOCALAPPDATA%\Microsoft\WinGet\Packages
Portable Package Root              C:\Program Files\WinGet\Packages
Portable Package Root (x86)        C:\Program Files (x86)\WinGet\Packages
Installer Downloads                %USERPROFILE%\Downloads

Links
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License Agreement   https://aka.ms/winget-license
Third Party Notices https://aka.ms/winget-3rdPartyNotice
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Admin Setting                             State
--------------------------------------------------
LocalManifestFiles                        Enabled
BypassCertificatePinningForMicrosoftStore Disabled
InstallerHashOverride                     Disabled
LocalArchiveMalwareScanOverride           Enabled

Start by comparing the root-level and installer-level ElevationRequirement examples in the linked pull requests and the schema documentation at doc/manifest/schema/1.6.0/installer.md. Confirm whether validation or Windows Package Manager behavior is responsible; done means the documented nested form validates and installs without elevation, or the schema and manifests clearly document the supported placement.