Bump MvvmCross.Plugin.Messenger from 6.4.2 to 7.0.0 (#24)

Bumps [MvvmCross.Plugin.Messenger](https://github.com/MvvmCross/MvvmCross) from 6.4.2 to 7.0.0.
- [Release notes](https://github.com/MvvmCross/MvvmCross/releases)
- [Changelog](https://github.com/MvvmCross/MvvmCross/blob/develop/CHANGELOG.md)
- [Commits](MvvmCross/MvvmCross@6.4.2...7.0.0)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

CastIt/CastIt.csproj

@@ -32,7 +32,7 @@
     <PackageReference Include="Microsoft.Xaml.Behaviors.Wpf" Version="1.1.19" />
     <PackageReference Include="MvvmCross" Version="7.0.0" />
     <PackageReference Include="MvvmCross.Platforms.Wpf" Version="6.4.2" />
-    <PackageReference Include="MvvmCross.Plugin.Messenger" Version="6.4.2" />
+    <PackageReference Include="MvvmCross.Plugin.Messenger" Version="7.0.0" />
     <PackageReference Include="MvvmCross.Plugin.ResxLocalization" Version="7.0.0" />
     <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
     <PackageReference Include="Serilog.Enrichers.Thread" Version="3.1.0" />