Adjust async/task/value task tests to work on net6 and on release branch

[wallymathieu]

We currently see that some tests fail on the v1.6, v1.7 branch.

• tests/FSharpPlus.Tests/ValueTask.fs the tests: testValueTaskZip, testValueTaskZipAsync
• tests/FSharpPlus.Tests/Asyncs.fs the tests: testAsyncZip, testAsyncZipAsync
• tests/FSharpPlus.Tests/Task.fs the tests: testTaskZip, testTaskZipAsync