Bumps Omnibus builder image to 4.3.1

What does this MR do?

Bumps the omnibus builder image version to 4.3.1 so that gcloud/gsutil is available on all builder images.

For https://gitlab.com/gitlab-org/security/omnibus-gitlab/-/issues/3, where we plan to switch from s3 to gcs for downloading packages.

diff: https://gitlab.com/gitlab-org/gitlab-omnibus-builder/-/compare/4.1.0...4.3.1?from_project_id=145205&straight=false

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened