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I think there is a bit of confusion on your part, you seem to be confusing the company Canonical who make Ubuntu with the English meaning of the word, it is an easy mistake to make here, a relevant definition of the word (from dictionary.com via Zanna in comments)

authorized; recognized; accepted:

So what they actually want is a definitive or best answer to the question rather than an answer from the developers.

This is a common term through out Stack Exchange, my explanation is not great so here are some examples from various sites

https://meta.stackoverflow.com/questions/291992/what-is-a-canonical-question-answer-and-what-is-their-purpose

Meaning of "Canonical answer required" bounty category

https://english.meta.stackexchange.com/questions/7115/what-is-a-canonical-answer

https://arduino.meta.stackexchange.com/questions/1269/what-is-a-canonical-question-exactly