github/workflows: Drop EOL versions and added latest 1.20

[jpds]

• Dropped end-of-life 1.17 and 1.18.
• Added 1.20.

[igungor]

There're a couple of places that needs to be updated :)

• README: we mention that min required version is 1.17+.
• Dockerfile: base image is 1.19-alpine. we could switch to 1.20 for Docker.
• go.mod: minimum required Go version is 1.17.

[igungor]

Thank you!

[igungor]

There're a couple of places that needs to be updated :)

• README: we mention that min required version is 1.17+.
• Dockerfile: base image is 1.19-alpine. we could switch to 1.20 for Docker.
• go.mod: minimum required Go version is 1.17.

I'll update remaining places in a separate PR.