Removing WCT 7.x references (#41733)

## Summary of the Pull Request

This PR introduces the following changes:
- Replace the MarkdownTextBlock with the latest version in
CommunityToolkit Labs, and removing the 7.x version.
- Replacing WrapPanel from 7.x with the 8.0 version.
- Replacing converters from 7.x with the 8.0 version.
- Remove unused namespaces related to the Toolkit

No visual or behavior changes, except for the release notes that now
look better :):

Before:
<img width="678" height="906" alt="image"
src="https://github.com/user-attachments/assets/8b3ac267-b4cd-499c-8e16-d8420a176a4a"
/>


After:
<img width="846" height="881" alt="image"
src="https://github.com/user-attachments/assets/cb4f2d85-0c23-4263-80d6-28c2ab403704"
/>


<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [ ] Closes: #xxx
- [ ] **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

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

---------

Co-authored-by: Gordon Lam <73506701+yeelam-gordon@users.noreply.github.com>
Co-authored-by: Gordon Lam (SH) <yeelam@microsoft.com>

Directory.Packages.props

@@ -1,6 +1,7 @@
 <Project>
   <PropertyGroup>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
+    <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="AdaptiveCards.ObjectModel.WinUI3" Version="2.0.0-beta" />
@@ -21,11 +22,11 @@
     <PackageVersion Include="CommunityToolkit.WinUI.Converters" Version="8.2.250402" />
     <PackageVersion Include="CommunityToolkit.WinUI.Extensions" Version="8.2.250402" />
     <PackageVersion Include="CommunityToolkit.WinUI.UI.Controls.DataGrid" Version="7.1.2" />
-    <PackageVersion Include="CommunityToolkit.WinUI.UI.Controls.Markdown" Version="7.1.2" />
-    <PackageVersion Include="CommunityToolkit.Labs.WinUI.Controls.MarkdownTextBlock" Version="0.1.250703-build.2173" />
+    <PackageVersion Include="CommunityToolkit.Labs.WinUI.Controls.MarkdownTextBlock" Version="0.1.250910-build.2249" />
     <PackageVersion Include="ControlzEx" Version="6.0.0" />
     <PackageVersion Include="HelixToolkit" Version="2.24.0" />
     <PackageVersion Include="HelixToolkit.Core.Wpf" Version="2.24.0" />
+    <PackageVersion Include="HtmlAgilityPack" Version="1.12.3" />
     <PackageVersion Include="hyjiacan.pinyin4net" Version="4.1.1" />
     <PackageVersion Include="Interop.Microsoft.Office.Interop.OneNote" Version="1.1.0.2" />
     <PackageVersion Include="LazyCache" Version="2.4.0" />