back compat: remove `--on-resume`

[oliver-sanders]

Remove this compatibility interface:

cylc-flow/cylc/flow/commands.py

Lines 656 to 660 in 1e45d07

	# BACK COMPAT: on_resume
	# Arg no longer used but retained for older clients.
	# From: 8.6
	# Remove at: 8.7
	on_resume: bool = False

And any CLI / GraphQL interfaces that use it, making sure not to cause compat issues.