Make m2e-users mailing list read-only

Summary

The m2e-committers have agreed [1] to replace the m2e-user mailing list [2] by Githup-discussions in the M2E GH-repo [3].

Therefore please make the m2e-users mailing list read-only.

1. https://github.com/eclipse-m2e/m2e-core/issues/420
2. https://www.eclipse.org/lists/m2e-users/
3. https://github.com/eclipse-m2e/m2e-core/discussions