That's completely beyond the scope of tgis conversation. We are talking about which franchise used the term "dragonborn" first
But that honestly doesn't matter.
The way copyright and trademark laws work (I am not a lawyer but I've run into the discussion enough), there are a few things that need to be met.
First, you need to actually put in the copyright paperwork. If they put in the paperwork to copyright it when they titled Tiber Septim (whoever that is) back in 2002 then there would have been a case already.
Secondly, you need to defend your copyright. This is one of the big reasons why you get "unreasonable" cease and desists from companies. If they do not pursue the defense of their copyright, they lose it, and it becomes public domain. (I'm sure it is far more complicated than that, being a legal thing, but it plays into the "there hasn't been a lawsuit" part)
And Finally, there are some things that you simply cannot copyright. And the term "dragonborn" as in "born of dragons" is probably one of them. You cannot copyright Dragon, they have existed in stories and art since before paper existed. Therefore, you could not copyright the idea of being born of a dragon, or having the blood of a dragon. In fact, warriors who have the blood or power of dragons go back to at least Sigurd of Germanic myth. Again, probably before the invention of paper.
They can't even copyright "warrior who has dragon blood and speaks the dragon language" because all of those things are non-copyrightable. You might as well try and copyright "warrior who is a prince and speaks to animals" it just isn't going to work.
So, it doesn't matter if the words "dragonborn" were first used by WoTC, TSR, or Bethesda. It is too generic a pairing of words to be copyrighted.