[Feature] Sunset .net core 3.1 

.NET Core 3.1 Will be end of support on December 13, 2022, that means we can start prepare the removal of support 

@asbjornu question, do you think we need to do this as part of v6.0 (we already removed support for .net 4.8 and net5.0) or release v6.0 and later do the removal of .net core 3.1 support