Bump github.com/go-acme/lego/v4 from 4.16.1 to 4.21.0 in /grid-proxy #5727

Job	Run time
build (activation-service)	8s
build-docker (activation-service)	28s
build (farmerbot)	7s
build-docker (farmerbot)	35s
build (grid-cli)	8s
build-docker (grid-cli)	17s
build (grid-proxy)	9s
build (gridify)	11s
build-docker (grid-proxy)	24s
build (monitoring-bot)	9s
build-docker (gridify)	18s
build (user-contracts-mon)	11s
build-docker (monitoring-bot)	35s
build (tfrobot)	4s
build-docker (user-contracts-mon)	17s
build-docker (tfrobot)	14s
	4m 15s

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