Reenable .NET Standard 2.0 builds for core library

[mregen]

Proposed changes

For some specific applications like analyzers it is preferable to have a .NET Standard 2.0 build of the core library.
Client/server build for NetStandard 2.0 will be maintained, but is unsupported in Nuget package and is not part of the test matrix.

Related Issues

• Fixes Do not remove support for .NET Standard 2.0 until really necessary #2559

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.53%. Comparing base (ebb55ad) to head (637a706).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2570      +/-   ##
==========================================
- Coverage   54.65%   54.53%   -0.12%     
==========================================
  Files         342      342              
  Lines       65044    65044              
  Branches    13331    13331              
==========================================
- Hits        35547    35473      -74     
- Misses      25648    25718      +70     
- Partials     3849     3853       +4     

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