chore: Set PackageLicenseExpression property to MIT

[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)
  

Related issues

• no releated issues

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

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[HofmeisterAn]

Thanks 💪.