CODE_OF_CONDUCT: replace Slack with Discord and Forum (#863)

Reflect the upstream change [1].

[1] exercism/org-wide-files@f72e90478cac

CODE_OF_CONDUCT.md

@@ -90,4 +90,4 @@ This policy was initially adopted from the Front-end London Slack community and
 A version history can be seen on [GitHub](https://github.com/exercism/website-copy/edit/main/pages/code_of_conduct.md).
 
 _This policy is a "living" document, and subject to refinement and expansion in the future.
-This policy applies to the Exercism website, the Exercism GitHub organization, any other Exercism-related communication channels (e.g.  Slack, Twitter, email) and any other Exercism entity or event._
+This policy applies to the Exercism website, the Exercism GitHub organization, any other Exercism-related communication channels (e.g. Discord, Forum, Twitter, email) and any other Exercism entity or event._