refactor: remove Terminal.Gui submodule and switch to NuGet package; update version to 0.2.7

src/Directory.Build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <Version>0.2.6</Version>
+    <Version>0.2.7</Version>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
   </PropertyGroup>

src/EchoHub.Client/EchoHub.Client.csproj

@@ -12,8 +12,7 @@
     <PackageReference Include="Serilog" Version="4.3.1" />
     <PackageReference Include="Serilog.Settings.Configuration" Version="10.0.0" />
     <PackageReference Include="Serilog.Sinks.File" Version="7.0.0" />
-    <!-- Using local fork until transparent color support is merged upstream (gui-cs/Terminal.Gui#4234) -->
-    <ProjectReference Include="..\Terminal.Gui\Terminal.Gui\Terminal.Gui.csproj" />
+    <PackageReference Include="Terminal.Gui" Version="2.0.0-develop.5039" />
   </ItemGroup>
 
   <ItemGroup>