Update Contributor Covenant from 1.4 to 2.1

[Pi-Cla]

The current COC comes from the 1.4 Contributor Covenant but ever since then the organization which wrote it has released two new versions so now the latest version is 2.1. Should we adopt this? I personally don't see why not, the changes seem reasonable enough.

Here is their changelog

[lpil]

Sounds sensible, let's do it!