Update CI, drop tests for old frameworks, add net7.0

[marcin-krystianc]

• Ubuntu-18.04 is deprecated ( "The Ubuntu-18.04 environment is deprecated and will be removed on April 1st, 2023" - The Ubuntu 18.04 Actions runner image will begin deprecation on 2022/08/08 and will be fully unsupported by 2023/04/03 actions/runner-images#6002). I think we should use "latest" images for all systems. Either "latest" or "fixed" images are being updated continuously, so I don't think there is a benefit of using a "fixed" version.
• netcoreapp2.1 and netcoreapp3.1 are now a history, I think it is enough to test the last three official releases.
• we need to use dotnet-format instead of dotnet format due to pending issues with the latter (dotnet format does not respect global.json and breaks when preview versions of .NET is installed dotnet/format#1500, System.Exception: Restore operation failed. dotnet/format#1560)