build!: require .NET 8 or later

[thomhurst]

Summary

• Retarget the library and test suite to net8.0;net9.0;net10.0.
• Remove the netstandard-only TaskCompletionSource shim, Microsoft.Bcl.AsyncInterfaces, and Polyfill.
• Collapse always-true NET6/NET8 conditional branches and wrappers; retain the NET9 completion-order split needed by net8.
• Document .NET 8 as the v4 minimum and list all tested TFMs.

Breaking change

v4 no longer supports net6.0 or netstandard2.0. Consumers must target .NET 8 or later.

Test plan

• dotnet restore TomLonghurst.EnumerableAsyncProcessor.sln --force-evaluate
• dotnet build TomLonghurst.EnumerableAsyncProcessor.sln -c Release --no-restore
• dotnet test TomLonghurst.EnumerableAsyncProcessor.sln -c Release --no-build (1,611 passed across net8.0, net9.0, net10.0)
• Package graph contains neither Polyfill nor Microsoft.Bcl.AsyncInterfaces
• Static search finds no removed TFM symbols/references; only the intentional NET9 conditional remains

Closes #334

[greptile-apps]

Greptile Summary

This PR raises the minimum supported runtime to .NET 8. The main changes are:

• Targets the library and tests at .NET 8, 9, and 10.
• Removes netstandard compatibility dependencies and conditional fallbacks.
• Enables async-enumerable examples and tests unconditionally.
• Documents the new supported frameworks.

Confidence Score: 5/5

This looks safe to merge.

• No qualifying blocking issues were found in the updated code.

Important Files Changed

Filename	Overview
EnumerableAsyncProcessor/EnumerableAsyncProcessor.csproj	Retargets the package to .NET 8, 9, and 10 and removes compatibility dependencies.
EnumerableAsyncProcessor/TaskCompletionSource.cs	Removes the netstandard-only task completion shim.
EnumerableAsyncProcessor/Validation/ValidationHelper.cs	Uses modern validation APIs directly on all supported targets.
EnumerableAsyncProcessor.UnitTests/EnumerableAsyncProcessor.UnitTests.csproj	Runs the test suite against .NET 8, 9, and 10.

_{Reviews (2): Last reviewed commit: "build!: require .NET 8 or later" | Re-trigger Greptile}