Bump MessagePack in /src/Platforms/Exceptionless.MessagePack

Bumps [MessagePack](https://github.com/MessagePack-CSharp/MessagePack-CSharp) from 2.5.108 to 2.5.187.
- [Release notes](https://github.com/MessagePack-CSharp/MessagePack-CSharp/releases)
- [Changelog](https://github.com/MessagePack-CSharp/MessagePack-CSharp/blob/master/prepare_release.ps1)
- [Commits](MessagePack-CSharp/MessagePack-CSharp@v2.5.108...v2.5.187)

---
updated-dependencies:
- dependency-name: MessagePack
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

src/Platforms/Exceptionless.MessagePack/Exceptionless.MessagePack.csproj

@@ -25,6 +25,6 @@
   </PropertyGroup>
 
   <ItemGroup Label="Package References">
-    <PackageReference Include="MessagePack" Version="2.5.108" />
+    <PackageReference Include="MessagePack" Version="2.5.187" />
   </ItemGroup>
 </Project>