Switch to actually maintained thingsboard pubsubclient

[thebentern]

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Our MQTT library of choice is without a maintainer and hasn't been updated in 4 years. Pretty depressing thread: https://github.com/knolleary/pubsubclient/issues/1045

Thingsboard has taken up the most involved fork of this and seems to have already integrated some good changes, even as recent as 2 weeks ago.