check-stable: Sort PRs by merge date
check-stable: Sort PRs by merge date

By the power of the dark art of the bourne again shell, sort the output
of the "check-stable" PR by merge date, oldest to newest. This should
ease backporting as sorting by PR number was inaccurate and would
sometimes reorder PRs, leading to more conflicts than necessary.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium,cilium-team/cilium