refactor(CancellableTasks): Delegate to Task.sequential/parallelLimit

[bartelink]

Follow up to #20128 as noted in #20128 (comment) - Replace bodies of CancellableTask.whenAllThrottled and sequential with Task.parallelLimit and sequential

[github-actions]

❗ Release notes required

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Change path	Release notes path	Description
`src/FSharp.Core`	docs/release-notes/.FSharp.Core/11.0.200.md	No release notes found or release notes format is not correct

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md

[Copilot]

Copilot review overview

🟡 Changes recommended

The sequential delegation can continue queued work after cancellation.

Review effort: Balanced
Findings: 1

Open (1)

• Retain per-iteration cancellation checks in sequential delegation · New

What changed in this PR

Refactors editor cancellable-task helpers to reuse FSharp.Core task combinators.

Changes:

• Delegates throttled parallel and sequential execution to Task helpers.
• Adds the follow-up PR to Visual Studio release notes.

File	Description
CancellableTasks.fs	Replaces custom scheduling implementations.
18.vNext.md	Adds the follow-up PR reference.

[bartelink]

could collapse this to fun ct -> Task.parallelLimit maxDegreeOfParallelism ct tasks (and same for sequential); no idea if it desugars to the same ?

[T-Gro]

Yes pls collapse it, lets use the FSharp.Core variants where possible.

[bartelink]

There's a call each of whenAllThrottled and sequential at present (neither has a ct locally so it makes sense for them to remain using it indirectly via the CancellableTask module)
I've removed the wrapping within the body to make them a one-liner.
Will eventually get IcedTasks to match it as part of the demystifyfp/FsToolkit.ErrorHandling#373 work