chore(deps): update dependency paho-mqtt to v1.6.1

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
paho-mqtt	==1.5.1 -> ==1.6.1

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[leahecole]

test failures are caused by leaked resources hitting quota

[leahecole]

adding blocked tag until IoT issues are resolved, which we're blocked on by cl/407918418

[dandhlee]

mqtt_example test was failing too much, just going to push this PR and investigate separately in a future update.