When we have an old cert that is being rotated, include it in the order.
If we're in the ARI-recommended rotation window, LE should exclude us
from rate limits. If we're not within that window, the order still
succeeds, so there's no risk in including the old cert.
Fixes#15542
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
463b47a0ab