Bump CommunityToolkit.Mvvm from 8.4.0-preview3 to 8.4.0 (#15)

Bumps [CommunityToolkit.Mvvm](https://github.com/CommunityToolkit/dotnet) from 8.4.0-preview3 to 8.4.0.
- [Release notes](https://github.com/CommunityToolkit/dotnet/releases)
- [Commits](CommunityToolkit/dotnet@v8.4.0-preview3...v8.4.0)

---
updated-dependencies:
- dependency-name: CommunityToolkit.Mvvm
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/SUBSTitute.csproj

@@ -96,7 +96,7 @@
   <!-- NuGet packages -->
   <ItemGroup>
     <PackageReference Include="CommunityToolkit.Labs.WinUI.TitleBar" Version="0.0.1-build.1830" />
-    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.0-preview3" />
+    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.0" />
     <PackageReference Include="CommunityToolkit.WinUI.Behaviors" Version="8.2.241204-build.1297" />
     <PackageReference Include="CommunityToolkit.WinUI.Collections" Version="8.2.241204-build.1297" />
     <PackageReference Include="CommunityToolkit.WinUI.Controls.Segmented" Version="8.2.241204-build.1297" />