Recently, many of the airlines start to use cabin crew instead of flight attendants. As an insider, I also know that cabin crew is a wider word that includes flight attendants, pursers, cabin in charge and so.
I see from your answer that you are a former flight attendant, so I don't doubt your ability to speak on the matter. My argument would be that Flight Attendant is still the more common term. The wikipedia page is called Flight attendant, the US union is called the Association of Flight Attendants, and every time I'm on a flight, I still hear "flight attendants, prepare for takeoff."
However, I am starting to hear the term "cabin crew" in common usage, and it seems entirely possible that it may supplant FA the same way FA supplanted stewardess sometime in the future. Or we might find that cabin crew becomes a term when speaking collectively, and FA is still used as a generic term for an individual in that position - the same way we have "flight crew" and captain and FO, but we still have the generic term pilot. I really have no idea.
My point is simply that the primary tag should probably reflect the most common name currently in use, and I believe that is flight attendant. Everyone knows what a flight attendant is; some people might be confused about the term cabin crew today. Granted, there is a grand total of three questions with the flight-attendant tag, so it's not exactly a high volume tag and whether we change it or not will affect the site very little.
As far as making them synonyms, I don't think anyone would object to that.