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
* work
* work
* work
* work
* work
* work
* react to feedback
* react to feedback
* .
* Update 20_21.md
* Update 20_21.md