Allow add only .NETStandard2.1 packages to curated list (w/o .NETStandard2.0 requirement)

[nick-dodonov]

Requirement "It must provide .NETStandard2.0 assemblies as part of its package" limits adding of useful packages.

Because of this, for example, popular and efficient MemoryPack (that is prepared to be used with Unity) refers to only NuGetForUnity competitor solution in its documentation for installation.
And it's required to raise self-hosted UnityNuGet instance to support deps on such packages now.

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I've also created Draft PR adding MemoryPack to list if requirement will be eased.