Support building with Go v1.16.9

What does this MR do?

Update the GO_VERSION in ci_files/variables.yml.

This is a security update: https://golang.org/doc/devel/release#go1.16

Closes https://gitlab.com/gitlab-org/charts/gitlab/-/issues/2951

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated