Replace ContextDeriver with context.WithoutCancel

[bluekeyes]

In Go 1.21, the context package got the context.WithoutCancel function, which creates a new context with all the same values as an existing context, but with an independent lifetime. This solves the same problem that I tried to solve with ContextDeriver.

It's easy enough to change the implementation of DefaultContextDeriver to use context.WithoutCancel and deprecate the other functionality, but I'd really like to remove this whole API. Internally, we don't use this at all and I couldn't find any public uses with Sourcegraph, so I think the impact would be minor. It should also be possible to move any custom functionality either into the handler or to a wrapper around the event dispatcher.