I believe Microsoft are still recommending people move to .NET 5 though. Unless you're extremely risk averse it seems like a very simple upgrade from 3.1 to 5 and hopefully 5 to 6 next November (which will be the LTS release) is the same.
I just did a straight up find-replace for `netcoreapp3.1` -> `net5.0` across 66 projects, built and passed tests without a hitch. Zero issues so far outside of the warnings resulting from enhanced nullable reference type smarts .