Code Quality: Drop support for .NET 6

[Lamparter]

.NET 6 lost support in November 2024. It should be removed from IronPython as it is not necessary to use anymore and most projects have moved to .NET 8 or .NET 9.

[BCSharp]

We have just dropped .NET Core 3.1! I think I'd rather wait until we support .NET 10.0, which should be somewhere at the end of 2025, so that we have always at least two .NET versions supported. .NET 9.0 would do too, but probably too much burden for our maintainer to stay up with all the STS versions.

It's hard to gauge what the most projects are running on, but judging from the sample of users' questions on this site, some are still on IronPython 2.x and .NET framework 3.5, so not everybody is moving fast forward.

[Lamparter]

Youch, though I imagine that's because the Windows release ships with the .NET Framework version of IronPython by default.