Heroku app stack deprecated notice

I received mail regarding one of our apps (App ID: 311):

Subject: “[eunice-crowdbotics-311] Your app’s stack is deprecated (Heroku-16)”

Your recently built app eunice-crowdbotics-311 is using the Heroku-16 stack, which is deprecated.

From May 1st 2021, Heroku-16 will be end-of-life, and apps using it will no longer receive security updates, and be run at your own risk.

From June 1st 2021, builds will no longer be allowed for Heroku-16 apps.

Please upgrade to a newer stack as soon as possible.

See the Heroku-16 End-Of-Life FAQ for more details.

What should be our next steps, if any?