Use Go 1.21.6

What does this MR do?

Use Go 1.21.6

- Update to Go 1.21.6

Closes https://gitlab.com/gitlab-org/gitlab-runner/-/issues/37357

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Why was this MR needed?

Go 1.21 update coming in 16.9

What are the relevant issue numbers?

Closes Build with Go 1.21.7 (#37357 - closed) • Robert Marshall • 16.9