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.