Bump Microsoft.CodeAnalysis.CSharp from 4.2.0 to 4.3.1

Bumps [Microsoft.CodeAnalysis.CSharp](https://github.com/dotnet/roslyn) from 4.2.0 to 4.3.1.
- [Release notes](https://github.com/dotnet/roslyn/releases)
- [Changelog](https://github.com/dotnet/roslyn/blob/main/docs/Breaking%20API%20Changes.md)
- [Commits](https://github.com/dotnet/roslyn/commits)

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

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

Directory.Packages.props

@@ -2,7 +2,7 @@
 
   <ItemGroup>
     <PackageVersion Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
-    <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="4.2.0" />
+    <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="4.3.1" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="1.1.1" />
     <PackageVersion Include="Nerdbank.GitVersioning" Version="3.5.113" />
     <PackageVersion Include="System.Memory" Version="4.5.5" />