Use PackageLicenseExpression in packages built from this repo

[KalleOlaviNiemitalo]

The SDK packages published from this repo include a LICENSE.txt file, which contains the text of the MIT license:

MSBuildSdks/Directory.Build.targets

Lines 27 to 28 in 5029011

	<PackageLicenseFile>LICENSE.txt</PackageLicenseFile>
	<PackageLicensePath>$(MSBuildThisFileDirectory)$(PackageLicenseFile)</PackageLicensePath>

However, https://docs.microsoft.com/en-us/nuget/reference/nuspec#license recommends that MIT-licensed packages should have <license type="expression">MIT</license> instead of referring to a file. In Directory.Build.targets, that would be <PackageLicenseExpression>MIT</PackageLicenseExpression>. I believe that would let the NuGet Gallery pages for Microsoft.Build.NoTargets and other packages display the text "MIT license", like the page for Microsoft.Extensions.Logging.Abstractions already does.

LICENSE.txt does contain a copyright notice that would not be covered by <license type="expression">MIT</license>:

MSBuildSdks/LICENSE.txt

Line 3 in 5029011

Copyright (c) Microsoft Corporation. All rights reserved.

However, almost the same copyright notice is already in the copyright element of the generated nuspec, so omitting the copyright notice from the license element seems OK. I believe it comes from the Copyright property that Directory.Build.targets sets:

MSBuildSdks/Directory.Build.targets

Line 26 in 5029011

<Copyright>© Microsoft Corporation. All rights reserved.</Copyright>

[jeffkl]

This sounds reasonable, I think when the packages were first published, NuGet did not have the feature of using an license expression. We should update to use this feature.