Update package license handling

[BruceHaley]

Fixes #5557

Description

This fixes "WARNING: The licenseUrl element is deprecated" on package pushes to Nuget.org. The licenseUrl element is in the .nuspec file in each Nuget package. The .nuspec file is autogenerated by Visual Studio when it does the packaging in the build.

This implements the recommended new way to handle licenses: reference them in licenses.nuget.org. (Or physically include them in the package.) Ours is the MIT license.

Specific Changes

Update Directory.Build.props replacing PackageLicenseUrl with PackageLicenseExpression.

[coveralls]

Pull Request Test Coverage Report for Build 252503

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-0.05%) to 78.856%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetNextViableTime.cs	1	90.91%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetPreviousViableTime.cs	1	90.91%
/libraries/Microsoft.Bot.Builder/DelegatingTurnContext.cs	1	88.24%
/libraries/Microsoft.Bot.Builder.Dialogs.Adaptive/Recognizers/EntityRecognizers/TextEntityRecognizer.cs	1	85.71%
/libraries/Microsoft.Bot.Builder.Dialogs/Prompts/DatetimePrompt.cs	1	77.5%
/libraries/Microsoft.Bot.Builder/Adapters/TestFlow.cs	2	78.32%
/libraries/Microsoft.Bot.Builder.Dialogs/Memory/DialogStateManager.cs	4	78.76%

Totals
Change from base Build 252410:	-0.05%
Covered Lines:	18177
Relevant Lines:	23051

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💛 - Coveralls