Is medical certification required for flight attendents?
One of the first flight attendants anywhere, Ellen Church was also a registered nurse and certified pilot. She was unable to find work as a commercial pilot but she did have another influence on the nascent flight attendant industry: she convinced a major airline to hire only nurses.

She approached BAT (the forerunner of United Airlines) looking for a pilot's job, a futile hope for women in those days. But the BAT exec did like Church's other suggestion: that commercial airliners carry nurses on board.
Smelling a publicity coup, and figuring that on-board nurses would help quell the public's fear – very real at the time – of flying, he sold her proposal to the boys at the top. BAT hired eight nurses, including Church, for what it thought would be a three-month experiment.

These early flights attendants bore little resemblance to their modern counterparts, however

They were expected to haul luggage, screw down loose seats, help with fueling the plane and finally, at day's end, help the pilots push the plane into the hangar.

In modern aviation, trying to hire a registered nurse as a flight attendant doesn't seem to be a requirement. Here's Delta's page on hiring and it says

While specific positions will have unique requirements (example: Flight Attendant minimum age requirement is 21 years of age; see individual job descriptions), you must possess at least a H.S. diploma/GED equivalent, be at least 18 years of age, and possess current authorization to work in the U.S. to work at Delta.

The individual job descriptions are for language knowledge. None list a nursing degree as a requirement.
What changed? Well, mainly the medical profession. Nurses today have a LOT more educational requirements. Additionally, a lot more places use nurses. So nurses are in high demand now. Adding that requirement onto a flight attendant job would make the position more expensive and harder to fill.
Additionally, there might be legal liability. While simple medical devices (like AED devices to restart a heart) supplement and can prove of some limited use, imagine a flight attendant having a nursing degree and someone has a medical emergency or dies on their aircraft. Suing the airline would be a tempting. Legal liability is a very real problem here

A colleague of mine, however, was not as fortunate and encountered a much more serious condition -- urosepsis -- on a flight to London. [snip] My colleague missed her connecting flight, as did many passengers, and the other physician had to accompany the patient to the local ER because the emergency medical service worker did not want to be held liable for the IV line the ICU physician had inserted before the plane landed.

And

Unfortunately, liability is a very real concern, particularly in scenarios when you're not practicing in your regular clinical setting. Generally, physicians are covered by the Aviation Medical Assistance Act of 1998, which protects physicians who provide in-flight emergency medical assistance in the same way that state Good Samaritan laws do. It goes without saying that you do not have to volunteer if you do not feel comfortable with the situation or are concerned for your own safety. If you have had a few drinks or have taken sedating medications, you need to use your judgment about whether to respond.
Since you are volunteering and not technically on duty, under the 1998 law, you are covered as long as you don't engage in any willful misconduct or commit gross negligence. You should make yourself aware of the law's limitations. Many physicians feel a moral obligation to help in a medical emergency, and, thus, we can find ourselves in these situations.

If passengers leap into action to help a distressed patent, the airline avoids liability.