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.
.NET 6 went into its first preview in the middle of February. Though it is not due for release until November, .NET Multi-platform App UI (MAUI) is worth taking a look a look at now. It is something to understand and get excited about, with a lot of changes coming.