Update dotnet-common-props metadata defaults

[ejball]

Summary

• make PackageProjectUrl and PackageReleaseNotes conditional defaults
• default Authors from GitHubOrganization only when Authors is unset
• stop managing PackageLicenseExpression in dotnet-common-props

Testing

• Invoke-Pester -Path .\conventions\dotnet-common-props\convention.Tests.ps1
• Parsed changed Directory.Build.props consumers as XML
• Ran git diff --check