Am I obligated to continue to work on a paper I contributed significantly to?

I think the Golden Rule applies here. While you have no obligation to act against your own self-interest here, if you were in your former collaborators' position, you would probably be rather unhappy at being blocked from publication because one co-author is no longer available.

Is your preference to not work on this paper at all, or just to limit the amount of time you spend on it? From your collaborators' point of view, a few hours of your time might be sufficient to avoid blocking the publication, while not placing much of a burden on you. It's perfectly OK to tell your collaborators, "I'd like to help, but I have many other demands on my time right now. I can devote 4 hours this Sunday to helping you get this publication ready, please let me know how I can use this time in a way that is most helpful to you."

On the other hand, if a few hours is really unmanageable to you right now, just tell them this, and let them know if you expect to have a little more time in the near future.

I suggest that you learn from this experience going forward, and make sure to document your work in sufficient detail for others to follow before you leave any research project.

I think you're wrong about it not being in your best interests. Having a paper from your undergraduate work will help you when you come to apply for jobs in the next stage of your career, and gaining experience of the writing and submission process now will help you when you come to publish papers from your PhD work and in writing up your thesis when it comes to that. Having work from more than one area published shows flexibility and is a useful asset if you want to move into a different area later.

Publishing your work is a key part of science, without it your work will be for nought. There is no hard obligation on you to publish but your previous advisor would be within his rights to be upset with you for walking out on your earlier work at this stage. If you're struggling for time, then explain the situation to your previous advisor and see whether you can reduce the time commitment needed while still providing the necessary work to get the paper published.

I don't think you have any formal obligations, and if you don't want to work on it because you have more pressing obligations, then don't work on it. It is usually a good idea to talk to the people you work with: tell them that you have little time to spare since you have a new job. That is perfectly reasonable, and reasonable people should understand and accept this. If your former co-workers want to publish, and you don't care, then they can do the work.

However, journal papers is academic gold. It always looks better with more papers on your publication list. Very few people actually read the papers when assessing your credentials; so even if the paper is in another domain, it will still look better on your CV. Something to consider.