Update branding to 2.1.6-servicing (#1994)

Per discussion with our team, we want to make the following changes to versioning during servicing updates:

* The versioning on packages with ship with ASP.NET Core should be consistent
* (No change) on packages with changes in them ship during servicing updates
* Version gaps are okay. For example, this repo is producing a patch for 2.1.6, so packages will be 2.1.6. It did not patch for 2.1.4 or 2.1.3 so those versions will be skipped.

This also includes another minor change: for consistency with the rest of the .NET Core teams, pre-release servicing builds will be 2.1.x-servicing-buildnumber.