Add support for old core frameworks versions from .net6

[yadanilov19]

Is your feature request related to a problem? Please describe.
I have problems on CI because I still using .net6 for my projects

Describe the solution you'd like
just update (if we could) target frameworks field in csprojs of refitter

Describe alternatives you've considered

Additional context

[christianhelle]

@yadanilov19 I have mixed feelings about supporting .NET 6.0 since it's no longer the recommended LTS version, and it's going out of support in November 2024

I never tried publishing .NET tool with multiple target frameworks before, but it's worth a shot. Let me see what I can do

[christianhelle]

@yadanilov19 I published a preview build to nuget.org that supports multiple target frameworks

I have pretty much all versions of .NET installed on my machine so it works out-of-the-box. Can you try?

[christianhelle]

@yadanilov19 if it works on your end then I'll merge it in and build a real release

[yadanilov19]

thx a lot!
I will test it immediately!

[yadanilov19]

Everything works fine with preview version!