[Settings] Optimizing and cleaning up code (#44721)

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fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request

- Removing dead experiments code
- Refactoring and simplifying OOBE/SCOOBE windowing code
- Removing dead assets
- Optimizing CmdPal assets to lower res shaving off a few MBs
- Scrolling works better on the What's new page
- Should be merged after: #44638

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## PR Checklist

- [X] Closes: #44958
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **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

---------

Co-authored-by: Gordon Lam (SH) <yeelam@microsoft.com>

src/settings-ui/Settings.UI/SettingsXAML/Views/CmdPalPage.xaml

@@ -28,7 +28,10 @@
                     <RowDefinition Height="Auto" />
                     <RowDefinition Height="Auto" />
                 </Grid.RowDefinitions>
-                <tkcontrols:OpacityMaskView Margin="-16,0,-16,0" HorizontalAlignment="Stretch">
+                <tkcontrols:OpacityMaskView
+                    Margin="-16,0,-16,0"
+                    HorizontalAlignment="Stretch"
+                    HorizontalContentAlignment="Stretch">
                     <tkcontrols:OpacityMaskView.OpacityMask>
                         <Rectangle>
                             <Rectangle.Fill>
@@ -40,17 +43,24 @@
                             </Rectangle.Fill>
                         </Rectangle>
                     </tkcontrols:OpacityMaskView.OpacityMask>
-                    <Grid Height="560">
+                    <Grid MaxHeight="560">
                         <Grid.RowDefinitions>
                             <RowDefinition Height="Auto" />
                             <RowDefinition Height="Auto" />
                             <RowDefinition Height="*" />
                         </Grid.RowDefinitions>
-                        <Image
-                            Grid.RowSpan="3"
-                            HorizontalAlignment="Stretch"
-                            Source="/Assets/Settings/Modules/CmdPal_Background.png"
-                            Stretch="UniformToFill" />
+                        <Grid.Background>
+                            <LinearGradientBrush StartPoint="0,0" EndPoint="1,1">
+                                <!--  Top-left: light cyan/blue  -->
+                                <GradientStop Offset="0.0" Color="#5FAFC9" />
+
+                                <!--  Mid transition  -->
+                                <GradientStop Offset="0.45" Color="#3E7FB0" />
+
+                                <!--  Bottom-right: deep blue  -->
+                                <GradientStop Offset="1.0" Color="#2C3E8F" />
+                            </LinearGradientBrush>
+                        </Grid.Background>
                         <TextBlock
                             x:Uid="CmdPal_HeroTitle"
                             Margin="0,24,0,12"
@@ -81,7 +91,7 @@
                             Margin="0,16,0,0"
                             HorizontalAlignment="Center"
                             VerticalAlignment="Top"
-                            Source="/Assets/Settings/Modules/CmdPal_Hero.png"
+                            Source="/Assets/Settings/Modules/CmdPal.png"
                             Stretch="Uniform" />
                     </Grid>
                 </tkcontrols:OpacityMaskView>