docs: add change log entry

changes/ce/fix-13012.en.md

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+The MQTT listerners config option `access_rules` has been improved in the following ways:
+
+* The listener no longer crash with an incomprehensible error message if a non-valid access rule is configured. Instead a configuration error is generated.
+* One can now add several rules in a single string by separating them by comma (for example, "allow 10.0.1.0/24, deny all").