Adopt new storage pickers from WinAppSDK 1.8 (#2044)

## Description
**Main changes:**
* Upgrade WinAppSDK package in **WinUIGallery** project to **1.8**
* Rename **File Picker** page to **Storage pickers** page for accuracy
(covers file & folder pickers)
* Rename corresponding sample image
* Add new examples with available picker options:
    * pick single file,
    * pick multiple files,
    * save file,
    * and pick folder
* Include a note explaining `SuggestedStartLocation` and `ViewMode`
behavior
* Update **RichEditBox** (3rd example) to use the new **Storage
Pickers** APIs
* Remove obsolete sample code files

## Motivation and Context
- Closes #2043 

## How Has This Been Tested?
Manually tested

## Screenshots (if appropriate):
<img width="1426" height="746" alt="image"
src="https://github.com/user-attachments/assets/64a314cc-3b16-41c8-8047-00e5a386b46b"
/>
<img width="1432" height="751" alt="image"
src="https://github.com/user-attachments/assets/8a8011fc-9c00-4c7a-9453-f5ab17f93f0e"
/>
<img width="1434" height="753" alt="image"
src="https://github.com/user-attachments/assets/8114d077-c23d-40bc-9337-23ba3495a134"
/>

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

---------

Co-authored-by: Niels Laute <niels.laute@live.nl>
Co-authored-by: Marcel Wagner <marcelwagner@microsoft.com>

Author: 3 people

WinUIGallery/Assets/ControlImages/FilePicker.png renamed to WinUIGallery/Assets/ControlImages/StoragePickers.png

@@ -39,7 +39,6 @@
     <Content Include="Assets\ControlImages\DropDownButton.png" />
     <Content Include="Assets\ControlImages\EasingFunction.png" />
     <Content Include="Assets\ControlImages\Expander.png" />
-    <Content Include="Assets\ControlImages\FilePicker.png" />
     <Content Include="Assets\ControlImages\FlipView.png" />
     <Content Include="Assets\ControlImages\Flyout.png" />
     <Content Include="Assets\ControlImages\Grid.png" />
@@ -235,16 +234,6 @@
     <Content Include="Samples\SampleCode\NavigationView\NavigationViewSample9_xaml.txt" />
     <Content Include="Samples\SampleCode\NumberBox\NumberBoxSample3_cs.txt" />
     <Content Include="Samples\SampleCode\NumberBox\NumberBoxSample3_xaml.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample1_cs.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample1_xaml.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample2_cs.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample2_xaml.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample3_cs.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample3_xaml.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample4_cs.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample4_xaml.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample5_cs.txt" />
-    <Content Include="Samples\SampleCode\System\FilePickerSample5_xaml.txt" />
     <Content Include="Samples\SampleCode\System\Window.txt" />
     <Content Include="Samples\SampleCode\SystemBackdrops\SystemBackdropsSampleBackdropTypes_cs.txt" />
     <Content Include="Samples\SampleCode\SystemBackdrops\SystemBackdropsSampleBackdropTypes_xaml.txt" />