Bump Go version to 1.21.9

What does this MR do?

Bump Go version to 1.21.9

- Use Go version 1.21.9 for regular and FIPS builds

Closes https://gitlab.com/gitlab-org/gitlab-runner/-/issues/37476

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

Why was this MR needed?

Regular maintenance.

What's the best way to test this MR?

What are the relevant issue numbers?

Closes Use Go 1.21.9 in GitLab Runner (#37476 - closed) • Robert Marshall • 17.0