Update .Net Core SDK to 2.2.300 to fix AppVeyor Ubuntu build

[bergmeister]

PR Summary

Not really clear what happened in the AppVeyor image or maybe a different behaviour of the new per-installed dotnet cli in it but the Ubuntu build started failing yesterday (no code change) in the bootstrapping code of the .Net SDK

This PR update the SDK version so that the Ubuntu build does not have to bootstrap it as a temporary measure since even verbose logs do not give much of a clue what goes wrong.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[bergmeister]

well, this did not go well, clearly a problem with a breaking behaviour change in the new .net sdk...

[bergmeister]

Re-open to test again