Drop `net6.0` and `net7.0` TFMs, keep `net8.0+` and `netstandard2.x`

[thomhurst]

Problem

TUnit currently targets netstandard2.1/net6.0+ with conditional compilation for different runtime capabilities. Both net6.0 (Nov 2024) and net7.0 (May 2024) are out of support.

Carrying these unsupported TFMs means maintaining #if branches for runtime features that exist in net8.0 but not net6.0/net7.0 (e.g. FrozenDictionary, improvements to UnsafeAccessor).

Proposed Change

• Drop net6.0 and net7.0 target frameworks
• Keep net8.0+ as the minimum modern .NET target
• Keep netstandard2.0 / netstandard2.1 for .NET Framework compatibility (still in support)
• Simplify #if blocks from net6.0 vs net7.0 vs net8.0 branches to netstandard vs net8.0+
• Enable net8.0+ features unconditionally on the modern path (e.g. FrozenDictionary, FrozenSet, SearchValues<T>)

Notes

• Breaking for anyone running TUnit on net6.0 or net7.0 — they'd need to upgrade to net8.0
• .NET Framework users are unaffected (covered by netstandard targets)
• Deferred to v2

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[Frulfump]

Not stale. .NET 8 and 9 goes out of support in November of this year but this change is good before that (and a little grace period afterwards of maybe 6 months)

[thomhurst]

I might just drop them in a minor version bump. Net standard should still let them work