cylc clean: improvements

[MetRonnie]

• Replace current --force option with --yes / -y - more apt (cylc clean: rename --force to --yes #4675)
  • Should --force come back in a different form? E.g. Allow cylc clean to work with --force when DB is corrupted/incompatible #4450
• Doing cylc clean toplevel and typing y at the interactive step for a workflow dir toplevel containing multiple runs, does not actually remove ~/cylc-run/toplevel at the moment (moved to Top level dir not removed when cleaning all runs #4963)
• Improve message for incompatible database - give instructions for how to manually delete db (Improve error message for incompatible DB #4876)
  • alternative to Allow cylc clean to work with --force when DB is corrupted/incompatible #4450

Pull requests welcome!