First up, what is the problem with split infinitives? Why must the adverb come AFTER the verb?
It's a myth. See, a couple centuries back, there were some pretentious grammarians who wanted to pretend that English was a Romance language and followed classy Latin grammar rules. So they made up Latin-based rules for English grammar and promoted them in books, irrespective of the fact that they didn't reflect how English had actually been spoken and written for centuries. And since the infinitive in Latin is a single word, they decided to treat an English infinitive phrase like "to go" as if it were one word, and thus insisted it had to be indivisible.
But this prescriptivist doctrine is absolutely wrong from a linguistic standpoint. The infinitive in English is not "to go," it's just "go." "To" is a helper word that goes with it, in the same way that "has" is a helper word in "has gone." So there's really no such thing as a split infinitive. Not unless you write "to gboldlyo" or something.
There are a lot of prescriptivist rules like this that just create a lot of unnecessary and awkward verbiage, like the one about not putting prepositions at the end of a sentence or not using "they" as a singular gender-neutral pronoun -- all of which were standard English usage for centuries before a few people invented rules saying they weren't proper English.
BTW, has anyone else noticed before that the line is in iambic pentameter before?
But it isn't. There are two consecutive unstressed syllables there, "one has." If it were "to boldly go where none have gone before," that would be iambic pentameter -- exactly five feet of two syllables each, or ten syllables in all. But it's actually three iambs, an anapest, and another iamb, totalling eleven syllables.