[19.4] Bump gitlab-omnibus-builder version to 5.68.0

What does this MR do and why?

Bumps BUILDER_IMAGE_REVISION from 5.67.0 to 5.68.0 in gitlab-ci-config/variables.yml on 19-4-stable, updating the gitlab-omnibus-builder image version used for CI builds.

Why

Debian 11 (bullseye) is EOL. Its apt repositories have moved to archive.debian.org and the bullseye-security repository is no longer available. As a result, the Debian 11 builder images (debian_11 and debian_11_arm64) fail to build, which blocks Debian 11 package builds for 19.4 patch releases.

This was fixed in gitlab-omnibus-builder by gitlab-omnibus-builder!508 (merged), which:

• Switches the apt sources to archive.debian.org
• Drops the bullseye-security repository
• Pins/downgrades packages that are no longer available in the archive
• Moves the distribution_ci_tools and ruby_kaniko images to Debian Bookworm

Why bump to 5.68.0 instead of backporting to a 5-67-stable branch

5.68.0 (gitlab-omnibus-builder@d807b13b) is exactly 5.67.0 (gitlab-omnibus-builder@1961e83c) plus the single fix from gitlab-omnibus-builder!508 (merged). There are no other changes between the two tags, so bumping directly is functionally identical to a stable-branch backport with no additional risk, and avoids creating a new 5-67-stable branch for a one-commit delta.

Related backports (older stable branches)

The same fix is being backported to the builder stable branches used by older GitLab versions:

• gitlab-omnibus-builder!513 (merged) (5-65-stable, GitLab 19.3)
• gitlab-omnibus-builder!514 (merged) (5-63-stable, GitLab 19.2)
• gitlab-omnibus-builder!515 (merged) (5-60-stable, GitLab 19.0 and 19.1)
• gitlab-omnibus-builder!516 (merged) (5-52-stable, GitLab 18.11)

Related issues

• gitlab-omnibus-builder!508 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).