Bump golang from v1.20 to v1.21

What does this MR do?

• Upgrades go.mod from 1.20 to 1.21
• Updates builder image in the Dockerfile

Notes

Official golang images for 1.21 stopped supporting buster (aka Debian 10) images and started supporting bookworm (aka Debian 12). This means we need to update both he builder imager and the main image of our Dockerfile so that we can satisfy the linker dependency (glibc).

Testing

Tested registry.gitlab.com/gitlab-org/security-products/analyzers/trivy-k8s-wrapper/nilieskou-upgrade-go-1-21:current built by the build branch job through the gitlab-agent.

What are the relevant issue numbers?

• Composition Analysis - Reaction Rotation for 16.10 (gitlab-org/secure/general#297 - closed)