As other replies have indicated, this is typically a problem with using a no-longer-supported OS (I mean one that the OS developer no longer provides updates for) or neglecting to apply OS updates.
If you administer a server for a client it’s really important to ensure that it’s getting software updates so that it doesn’t get hacked and become part of a botnet, get turned into blackhat SEO spam, or leak customer data from the server. If you don’t know how to make this is happening as a web designer, you should consider using a different style of hosting plan so that there is someone who will take responsibility for that aspect.
(We’re happy to help with this kind of problem here on the forum—that’s what we’re here for!—but “make sure that your web server is getting regular software updates” may be the ultimate solution for most people who see this message.)
(This particular change was announced more than one year ago, and, for most Certbot users’ use cases though not all, was already fixed in Certbot releases at that time. I know that not all users of Let’s Encrypt certificates follow, or have any reason to follow, upcoming policy or technical changes in Let’s Encrypt services, which is totally fine. But if you don’t, your Let’s Encrypt client application developer is following them for you and acting on them for you, which means you need to have a path to update to a new Let’s Encrypt client application release when it comes out. This is broadly true for most kinds of Internet software—your upstream software developer is following problems and changes that need to be acted on, and periodically making new version releases that will address those.)