Update go.mod and Prometheus version

Changelog for 0.18.3:

• PR #347 Bump google.golang.org/grpc from 1.53.0 to 1.56.3 by @dependabot[bot]
• PR #352 mark namespace with label openfaas=1 valid by @nitishkumar71

Commits
f17a25f Update go.mod and Prometheus version by @alexellis
7ef56d8 Bump google.golang.org/grpc from 1.53.0 to 1.56.3 by @dependabot[bot]
1cb5493 mark namespace with label openfaas=1 valid by @nitishkumar71
d85332b Update queue-worker, NATS, Prometheus and gateway by @alexellis
1412faf Add Makefile target for updating images by @alexellis
2685c1d Upgrade to go-execute/v2 by @alexellis

Changes: 0.18.2...0.18.3

Generated by Derek