Bump MongoDB driver to 2.28.0

[miszu]

Thanks a lot for maintaining the library, appreciate it!

I bumped the driver to the latest v2.28.0 (https://github.com/mongodb/mongo-csharp-driver/releases/tag/v2.28.0). I was not able to run the tests locally on my Mac due to missing NET4.7, so I spent some time updating the setup. Some of these changes are a bit opinionated, so I'll totally understand if you'd prefer to skip some changes (feel free to give feedback or make any edits yourself).

• changed target framework for test projects to net8
• removed App.config from CoreIntegrationTests, as it was not used there
• in IntegrationTests, I couldn't get the App.config to work, so I migrated to appsettings.json
• simplified IntegrationTests.csproj by removing not needed references

Green tests 🎉