pallet-bounties: allow bounties to never expire #17200

Job	Run time
isdraft / isdraft	1s
preflight / preflight	5s
preflight / ci-versions	1m 13s
build-linux-stable	10m 19s
build-linux-stable-cumulus	12m 26s
build-malus	8m 41s
build-test-collators	7m 25s
build-test-parachain	5m 7s
prepare-bridges-zombienet-artifacts	8m 39s
build-linux-substrate	6m 44s
prepare-cumulus-zombienet-artifacts	2m 50s
prepare-polkadot-zombienet-artifacts	6m 45s
build-push-image-test-parachain	54s
build-push-image-substrate-pr	56s
build-push-image-colander	2m 3s
build-push-image-malus	2m 5s
build-push-image-polkadot-debug	2m 19s
build-push-image-polkadot-parachain-debug	1m 17s
build-push-image-bridges-zombienet-tests	4m 0s
	1h 23m 49s

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