Upgrade Durable V3.x to use .NET 6 Framework only

[nytian]

Issue describing the changes in this PR

• [x] Update WebJobs.Extensions.DurableTask to use dotnet 6.0 only
• [x] Remove all #if !Functions v1 section
• [x] Remove all #if FUNCTIONS_V2_OR_GREATER, which is eqult to #if !Functions v1
• [x] Remove tests about Smoke Tests Functions v1, v2 and v3. And update tests to use Azure Functions Runtime v4.
• [x] Remove #if Functions_V2 or greater and #ifFunctions_V3 or greater since DF v3 will only support Functions v4.

Pull request checklist

• [ ] My changes do not require documentation changes
  • [ ] Otherwise: Documentation PR is ready to merge and referenced in pending_docs.md
• [ ] My changes should not be added to the release notes for the next release
  • [ ] Otherwise: I've added my notes to release_notes.md
• [ ] My changes do not need to be backported to a previous version
  • [ ] Otherwise: Backport tracked by issue/PR #issue_or_pr
• [ ] I have added all required tests (Unit tests, E2E tests)
• [ ] My changes do not require any extra work to be leveraged by OutOfProc SDKs
  • [ ] Otherwise: That work is being tracked here: #issue_or_pr_in_each_sdk
• [ ] My changes do not change the version of the WebJobs.Extensions.DurableTask package
  • [ ] Otherwise: major or minor version updates are reflected in /src/Worker.Extensions.DurableTask/AssemblyInfo.cs
• [ ] My changes do not add EventIds to our EventSource logs
  • [ ] Otherwise: Ensure the EventIds are within the supported range in our existing Windows infrastructure. You may validate this with a deployed app's telemetry. You may also extend the range by completing a PR such as this one.
• [ ] My changes should be added to v3.x branch.
  • [ ] Otherwise: This change only applies to Durable Functions v2.x and will not be merged to branch v3.x.

[nytian]

The Functions v1 tests CI fail because V1 tests were deleted in this PR.

[davidmrdavid]

@jviau would be good if you could skim over this. It LGTM for the most part, but would appreciate another set of eyes.

[jviau]

This looks good to me. Will need a lot of manual validation to ensure all scenarios still work.