CmdPal: Replace the brush used for the menu item separator (#41130)

## Summary of the Pull Request

Replace the brush used for the menu item separator in
SeparatorContextMenuViewModelTemplate with the brush used by WinUI 3 for
flyout menus.

The brush previously used is a legacy brush and a WinUI trap.

After screenshot:

<img width="830" height="513" alt="image"
src="https://github.com/user-attachments/assets/32620050-29a3-40a5-aa6a-fe83afb55090"
/>


## PR Checklist

- [x] Closes: #41128
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/ContextMenu.xaml

@@ -112,7 +112,7 @@
                 <Rectangle
                     Height="1"
                     Margin="-16,-12,-12,-12"
-                    Fill="{ThemeResource MenuFlyoutSeparatorThemeBrush}" />
+                    Fill="{ThemeResource MenuFlyoutSeparatorBackground}" />
             </DataTemplate>
         </ResourceDictionary>
     </UserControl.Resources>