Samples: Upgrade go to 1.13 and use go modules

[grayside]

Go samples specify golang, golang:1.11, or golang:1.12.

Describe the change you'd like to see

Following the stable release of Go 1.13 with a container image on Docker Hub, samples using Go should go through the following:

• Upgrade samples to use golang:1.13
• Switch to use modules by default, matching the behavior of the go toolchain. This includes adding a go.mod and go.sum.
• Case by case evaluation if other language changes should be applied.

[grayside]

go 1.13 is now available.

[grayside]

The Dockerfile for helloworld-go should be used as a template for all go samples.

[knative-housekeeping-robot]

Issues go stale after 90 days of inactivity.
Mark the issue as fresh by adding the comment /remove-lifecycle stale.
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/lifecycle stale

[grayside]

/remove-lifecycle stale

[knative-housekeeping-robot]

Issues go stale after 90 days of inactivity.
Mark the issue as fresh by adding the comment /remove-lifecycle stale.
Stale issues rot after an additional 30 days of inactivity and eventually close.
If this issue is safe to close now please do so by adding the comment /close.

Send feedback to Knative Productivity Slack channel or file an issue in knative/test-infra.

/lifecycle stale