Bump Microsoft.Extensions.Configuration in /src/STIL.ServiceClient.Tests

Bumps [Microsoft.Extensions.Configuration](https://github.com/dotnet/runtime) from 7.0.0 to 8.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v7.0.0...v8.0.0)

---
updated-dependencies:
- dependency-name: Microsoft.Extensions.Configuration
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

src/STIL.ServiceClient.Tests/STIL.ServiceClient.Tests.csproj

@@ -33,7 +33,7 @@
 
   <ItemGroup>
     <PackageReference Include="FluentAssertions" Version="6.12.0" />
-    <PackageReference Include="Microsoft.Extensions.Configuration" Version="7.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Configuration" Version="8.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
 
     <PackageReference Include="Moq" Version="4.20.69" />