Bump github.com/go-acme/lego/v4 from 4.16.1 to 4.21.0 in /grid-proxy #5723

Job	Run time
build (activation-service)	6s
build-docker (activation-service)	28s
build (farmerbot)	4s
build-docker (farmerbot)	32s
build (grid-cli)	3s
build-docker (grid-cli)	18s
build (grid-proxy)	8s
build-docker (grid-proxy)	22s
build (gridify)	8s
build-docker (gridify)	17s
build (monitoring-bot)	4s
build-docker (monitoring-bot)	40s
build (user-contracts-mon)	3s
build-docker (user-contracts-mon)	16s
build (tfrobot)	9s
build-docker (tfrobot)	15s
	3m 53s

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