Update the current Contributor's Code of Conduct

In the current Code of Conduct, we direct users to report issues to a specific e-mail address. Instead, we should use an e-mail alias which redirects to the appropriate parties.

The file in question:
https://github.com/Graylog2/graylog2-server/blob/master/CODE_OF_CONDUCT.md

A new e-mail address, code-conduct@graylog.com, should be used under the "Enforcement" section.

In addition to the above, the Contributor Covenant, which our Code of Conduct is based upon, has been updated a bit since the last time we visited this - we should look to adopt the newer language as well to stay up to date:
https://www.contributor-covenant.org/version/2/1/code_of_conduct/