Merge 2.1.x down to master and shut down 2.1.x

Now that Puppet Server 2.0 has been released, we should, assuming the 2.1.x branch CI jobs are all green, act on the latest plan from our branching document:

1) Do a non-fast forward merge up of the 2.1.x branch to the master branch, edit the README_BRANCHING.md document to indicate that only work toward 2.1 should be going to master right now and remove references to the 2.1.x branch. Submit a PR for that work up to the master branch.

2) Submit a separate PR to the stable branch with the same set of changes to README_BRANCHING.md as were submitted to the master branch.

3) Once the PR to master is merged and everything's green through CI, work with QE to delete the 2.1.x pipeline - probably involving the submission of a QE ticket with said intent - and subsequently delete the 2.1.x branch from the repo.