Bump Microsoft.CodeAnalysis.CSharp from 4.2.0 to 4.3.0 in /backend

Bumps [Microsoft.CodeAnalysis.CSharp](https://github.com/dotnet/roslyn) from 4.2.0 to 4.3.0.
- [Release notes](https://github.com/dotnet/roslyn/releases)
- [Changelog](https://github.com/dotnet/roslyn/blob/main/docs/Breaking%20API%20Changes.md)
- [Commits](dotnet/roslyn@v4.2.0...Visual-Studio-2019-Version-16.0-Preview-4.3)

---
updated-dependencies:
- dependency-name: Microsoft.CodeAnalysis.CSharp
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/RossyntBackend/RossyntBackend.csproj

@@ -7,7 +7,7 @@
 
     <ItemGroup>
       <PackageReference Include="JetBrains.Annotations" Version="2022.1.0" PrivateAssets="All" />
-      <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.2.0" />
+      <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.3.0" />
       <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     </ItemGroup>