• mqtt: stop including mqtt_protocol.h on mosquitto 2.1+

    From Rob Swindell@VERT to GitLab note in main/sbbs on Fri Sep 11 15:16:44 2026
    https://gitlab.synchro.net/main/sbbs/-/merge_requests/732#note_10330

    We don't normally include "GitLab" in references to issue numbers.

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