Code of conduct update to v2.1

[sverhoeven]

Checklist before submitting an issue

• I have checked the existing issues and couldn't find an issue about this bug.

Issue details

The CODE_OF_CONDUCT.md and template/CODE_OF_CONDUCT.md are version 1.4 of the Contributor Covenant.

There is version v2.1 of the Contributor Covenant. It would be nice if the template uses the latest version.

[sverhoeven]

Now there is v3 at https://www.contributor-covenant.org/version/3/0/code_of_conduct/