set: implement the new command

[oliver-sanders]

Implement accepted proposal A New Command for Manual Task Forcing.

• Rename set-outputs to set.
• Implement the logic for setting prereqs/outputs as per the proposal.
• Update the CLI and GraphQL schema (test this works as expected in the ui).
• Update documentation.
• This is a breaking change, add an entry into the cylc-doc changes page.

[hjoliver]

I'm currently working on this.

Subject to discussion, I've changed the new command name to cylc set-task.

• there's a CLI ambiguity problem (set-verbosity)
• plain set is arguably not specific enough