Forbid creating new fleets on run apply

[r4victor]

Part of #2973

Currently, dstack fleets are optional for cloud backends, which means users have two ways of provisioning via dstack: using fleet configurations or run configurations. The proposal is to make provisioning UX fleet-first: Run configurations won’t be able to create new fleets but will only use fleets explicitly configured by users. Thus, all provisioning use cases will be unified with the same UX.

Runs that don't need clusters may use one default elastic fleet:

type: fleet
name: default-fleet
nodes: 0..

Multi-node tasks require creating cluster fleets explicitly.

Plan

1. Display a CLI warning if a run creates fleet informing that future dstack version will require creating fleets explicitly.
2. Forbid auto-created fleets and display a CLI error if there is no eligible fleet for the run. For non-multi-node runs, also suggest creating default elastic fleet.

[github-actions]

This issue is stale because it has been open for 30 days with no activity.