Update golang version used in App Engine

[SaitejaTamma]

Is your feature request related to a problem? Please describe.
Update the go runtime used in App Engine for the website.

Describe the solution you'd like
Updating the version of go will remove the warning printed while running make site-deploy during the release process.

Describe alternatives you've considered
Ignore the warning until things break.

Additional context
WARNING: Go 1.13 is no longer supported by the Go community as of August 2020. We recommend you to upgrade to the latest version of Go runtime as soon as possible. For details on upgrading, see https://cloud.google.com/appengine/docs/standard/go/runtime.

[markmandel]

For context: Which make command did you run that resulted in this message?

[roberthbailey]

Offending line:

agones/site/app.yaml

Line 15 in ec15511

runtime: go113

[roberthbailey]

I updated the issue with more context. This happens during the release process while deploying the website.

[roberthbailey]

From https://cloud.google.com/appengine/docs/standard/go/runtime it looks like we can choose go 1.12, 1.13, 1.14, 1.15, or 1.16.

Given that we've been using 1.13 for a long time and it's been deprecated for a year and a half, I think we should jump to 1.16 (unless that breaks something) to give ourselves the maximum amount of time before we need to update again.