Explicitly set license for nuget through PackageLicenseExpression

[ax-meyer]

What does this PR do?

Switches from using the license file to indicate the license in the nuget package to using PackageLicenseExpression to explicitly define the license for the nugets.

Why is it important?

This allows

• Allows tools like nuget-license (https://github.com/sensslen/nuget-license) to automatically detect the license of a package - currently we need to configure manual overrides and update the version in the override each time testconatainers gets updated
• nuget.org to show the license in the sidebar directly like this (see here: https://www.nuget.org/packages/dotnet-ef)
  

How to test this PR

Manual test:

• Run nuget pack for any project in the repo
• Start manual upload process for a nuget here: https://www.nuget.org/packages/manage/upload
• The page will show the package metadata with license information for verification before finishing the upload

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.30%. Comparing base (9f6dc48) to head (a7f2631).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1095   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files         113      113           
  Lines        3448     3448           
  Branches      971      971           
=======================================
  Hits         3217     3217           
  Misses        215      215           
  Partials       16       16           

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