Para upgrade blocks offboarding

[slumber]

This restriction was introduced in #4360:

polkadot/runtime/parachains/src/paras/mod.rs

Lines 1573 to 1582 in 28acc78

	// Disallow offboarding in case there is an upcoming upgrade.
	//
	// This is not a fundamential limitation but rather simplification: it allows us to get
	// away without introducing additional logic for pruning and, more importantly, enacting
	// ongoing PVF pre-checking votes. It also removes some nasty edge cases.
	//
	// This implicitly assumes that the given para exists, i.e. it's lifecycle != None.
	if FutureCodeHash::<T>::contains_key(&id) {
	return Err(Error::<T>::CannotOffboard.into())
	}

However, runtime only processes an upgrade when a para progresses, if it never does after receiving a GoAhead signal, then there's no way to offboard it.

What actually I suppose was meant in a mentioned PR is to disallow offboarding while there's an ongoing pvf prechecking process, which is totally fine, it finishes rather quickly or at worst times out.

[slumber]

cc @pepyakin

[pepyakin]

Yes, good call!