Update dotNET-Core.gitlab-ci.yml template image

Ole Jørgen Skogstadrequested to merge
softis-software/gitlab:softis-software/update-dotnet-core-ci-template into master

What does this MR do and why?

Microsoft has moved their repositories on docker hub, pulling microsoft/dotnet:latest results in an error.

Updated image to mcr.microsoft.com/dotnet/sdk:latest, and Microsofts docker hub URL to https://hub.docker.com/_/microsoft-dotnet.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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