README: remove mailing list

The mailing list was sunset by the IT department and
was rarely used, so we'll replace it with matrix & discussions.

README.md

@@ -14,8 +14,7 @@ Image Builder is intended to be run within the
  * **Website**: <https://www.osbuild.org>
  * **Bug Tracker**: <https://github.com/osbuild/image-builder/issues>
  * **Discussions**: https://github.com/orgs/osbuild/discussions
- * **Matrix**: #image-builder on [fedoraproject.org](https://matrix.to/#/#image-builder:fedoraproject.org)
- * **Mailing List**: [email protected]
+ * **Matrix**: [#image-builder on fedoraproject.org](https://matrix.to/#/#image-builder:fedoraproject.org)
  * **Changelog**: <https://github.com/osbuild/image-builder/releases>
 
 ### OpenAPI spec