Fixes#9219
@JunTaoLuo Dan calls out the implicit versioning "for 2.1 SDK or later;" however, we don't usually version the doc content by SDK release. We might need to say something for this update in the passage that I'm adding to the All topic (*aspnetcore/fundamentals/metapackage.md*) about which SDK version this applies to. I don't know if that's something that you would like to do or what version to call out.
I also remove the version from the samples. We also assume that devs are using the latest SDK with our samples.
Follow up to https://github.com/aspnet/Docs/pull/7540
I found one more reference to versionless packages. I've updated the migration guide to remove this recommendation, and also briefly mentioned the most common issues users have encountered so far.
* WIP Migrate from 2.0 to 2.1
* work
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* react to feedback
* react to feedback
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* Update 20_21.md
* Update 20_21.md