Follow kessai's create() rename: pass deadline instead of ttl

kessai renamed PaymentManagerInterface::create()'s $ttl parameter to $deadline in kessai [#3614236], along with its default_payment_ttl config key. kessai is pre-1.0 and reinstall-only, so the rename lands outright rather than being aliased.

PaymentInteraction::startPayment() passes that argument by name, and a named argument that no longer exists is a fatal Error at call time, not a deprecation. So this breaks the moment kessai is updated.

One call site, plus the local variable and the comment above it, which also still said ttl. Nothing else in orchestra reads the kessai config key.

Should land together with the kessai change.