code-of-conduct.md: use CNCF code of conduct

t-lo · web-flow · commit 492f1546f3e8 · 2023-07-24T18:54:39.000+02:00
diff --git a/code-of-conduct.md b/code-of-conduct.md
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+# Code of Conduct
+
+We follow the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/main/code-of-conduct.md).
+
+Please contact [private Maintainer mailing list](maintainers@flatcar-linux.org) or the Cloud Native Foundation mediator, conduct@cncf.io, to report an issue.
