The end of long-term support for .NET Core 3.1 is due on the 13th of December 2022. The recommended move is direct to .NET 6.
It is important to remember that though apps will continue to run and function as normal on .Net Core 3.1, without support, there will be no updates. This includes all new security fixes being provided to .NET 6. Meaning .NET Core 3.1 will be open to vulnerabilities after the 13th of December.
If you have any questions about the .NET releases, you can see the releases and support dates here and the Microsoft policies that govern this cycle, here.