Remove mailing list mention in README.md (#6895)

Swift Forums are supposed to be mentioned instead, as the mailing list has been retired.

Author: MaxDesiatov

README.md

@@ -77,7 +77,7 @@ If you have any trouble with the package manager, help is available. We recommen
 
 When reporting an issue please follow the bug reporting guidelines, they can be found in [contribution guide](./CONTRIBUTING.md#reporting-issues).
 
-If you’re not comfortable sharing your question with the list, contact details for the code owners can be found in [CODEOWNERS](CODEOWNERS); however, the mailing list is usually the best place to go for help.
+If you’re not comfortable sharing your question with the list, contact details for the code owners can be found in [CODEOWNERS](CODEOWNERS); however, Swift Forums is usually the best place to go for help.
 
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