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

[thomhurst]

Summary

• Remove net6.0 from TargetFrameworks in TUnit.TestProject.Library and TUnit.NugetTester csproj files
• Update all 139 #if NET6_0_OR_GREATER and #if NET7_0_OR_GREATER preprocessor directives to #if NET8_0_OR_GREATER across 67 files
• .NET Framework users are unaffected (covered by netstandard2.0 targets)

Both net6.0 (Nov 2024) and net7.0 (May 2024) are out of support. The library projects already target netstandard2.0;net8.0;net9.0;net10.0 — this change aligns the preprocessor directives and test projects with that reality.

Closes #4786

Test plan

• Solution builds with 0 errors
• CI passes across all target frameworks (net8.0, net9.0, net10.0, netstandard2.0, net472)

[claude]

Code Review: Drop net6.0 and net7.0 TFMs

This is a clean, well-scoped maintenance PR. Both .NET 6 (EOL Nov 2024) and .NET 7 (EOL May 2024) are out of support, and aligning the preprocessor guards with the actual supported TFMs is the right call.

What's done well

• Mechanically correct: All 74 preprocessor directive changes (71× NET6_0_OR_GREATER + 3× NET7_0_OR_GREATER) are consistently updated to NET8_0_OR_GREATER, which matches the minimum specific TFM now supported.
• No functionality lost: Since net8, net9, net10 all satisfy NET8_0_OR_GREATER, all previously-guarded code (DateOnly/TimeOnly, Parallel.ForEachAsync, Task.IsCompletedSuccessfully, generic math, static abstract members, etc.) continues to compile and run for all supported consumers.
• Correct project file cleanup: TUnit.TestProject.Library and TUnit.NugetTester have their net6.0 targets removed; the main library projects already target netstandard2.0;net8.0;net9.0;net10.0, so no change needed there.
• netstandard2.0 users are unaffected: The netstandard2.0 target never defined these symbols, so the fallback code paths remain unchanged.

Minor observations (no action required)

• Feature-to-guard accuracy: A few of the original NET6_0_OR_GREATER guards were slightly conservative for their feature (e.g., Task.IsCompletedSuccessfully is available since .NET 5 / netstandard2.1, and BigInteger.IsPowerOfTwo predates .NET 6 entirely). Since the project doesn't target netstandard2.1 or net5, this was already moot before the PR and remains moot after — the new NET8_0_OR_GREATER guards are perfectly correct for the current TFM matrix.

• NET7_0_OR_GREATER → NET8_0_OR_GREATER: The three cases here (MatrixRangeAttribute<T> using INumber<T> generic math, StaticAbstractMemberTests.cs, and MatrixTests.cs) all require features introduced in .NET 7 / C# 11. With .NET 7 dropped, NET8_0_OR_GREATER is the correct minimum guard.

Verdict

Approved. The change is straightforward, correct, and a sensible step in keeping the project aligned with the supported .NET release cadence. Nothing to block on.

[codacy-production]

Up to standards ✅

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