Rename Overlord task "shutdown" to "cancel"

[leventov]

It seems to be an unfortunate name because "shutdown" sounds like a type of task itself, not a task cancellation request.

For retaining backward compatibility, old endpoints should still be supported (with a warning logged issued when they are called), but generally, it should be called "canceled" in new endpoints and throughout the documentation, codebase, and new HTTP endpoints.

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