Talk to your boss (manager, CTO, etc...) first! You have nothing to lose by doing this. Be completely honest with them about your feelings, but phrase things in positive ways. Don't say you "already stopped learning new things", but tell them you "have a lot more to offer and ready for more challenges".

Do you know nothing cool is coming down the pipeline for the foreseeable future? Show initiative and tell them all of the things that you genuinely love about the company and how you like it there. Don't put them on the defensive or focus on the negatives. During that conversation discuss the current positives and the future positives with you as an engaged employee with more responsibility.

You have absolutely nothing to lose if you are on the fence with your decision. If they react poorly you know what you need to do. If they react positively you may have a long-term happy future there.