csharp: Bump the csharp group with 3 updates

Bumps Microsoft.NET.Test.Sdk from 18.3.0 to 18.4.0
Bumps MSTest.TestAdapter from 4.1.0 to 4.2.1
Bumps MSTest.TestFramework from 4.1.0 to 4.2.1

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: csharp
- dependency-name: MSTest.TestAdapter
  dependency-version: 4.2.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: csharp
- dependency-name: MSTest.TestFramework
  dependency-version: 4.2.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: csharp
...

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

Author: dependabot[bot]

lang/csharp/test/sourcepp.test.csproj

@@ -17,9 +17,9 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
-    <PackageReference Include="MSTest.TestAdapter" Version="4.1.0" />
-    <PackageReference Include="MSTest.TestFramework" Version="4.1.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
+    <PackageReference Include="MSTest.TestAdapter" Version="4.2.1" />
+    <PackageReference Include="MSTest.TestFramework" Version="4.2.1" />
   </ItemGroup>
 
   <ItemGroup>