Create or Update Release Notes for dotnet 2021-03 release

[azure-sdk]

Please use this PR for 2021-03 dotnet notes update. Follow the guidelines below.

• Release manager should remove all entries for packages that should not be in this release period.

  • You will have to remove it from the Release Highlights, Installation Instructions as well as the Updates, Beta or, GA section as the case may be.
  • Please don't remove the entries inside the HTML comments <!-- --> as that will cause the automation to re-add the entry on the next run.
  • Feel free to edit individual Release Highlights sections as you see fit.

• Add new release entries to the PR if it has not already been added by the automation (it most likely will be).

  • Ensure to leave one line of space around (before and after) the special comments of the form [pattern.beta]: # (- ${PackageFriendlyName})
  • If there are packages that should be in this release that don't appear its probably because this automation runs at 12:30PM every weekday. If you released you package after 12:30PM you should wait till 12:30PM the next day and the automation would have picked it up.
  • To prevent the conflicts between the automation and manual edits, instead of pushing new commits you should make code suggestion on the PR, then allow the release manager to take care of merging everything in.

• Leave feedback letting us know what group of packages should be kept out of the releases as well as any other issues you notice.