Rename Overlord task "shutdown" to "cancel"

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.

Start by locating the Overlord task "shutdown" endpoints and all related references in the codebase and documentation. Update new HTTP endpoints and documentation to use "cancel" or "canceled", while keeping old endpoints supported with a logged warning when called.

Written by the indexing model from the issue text.