Backport 'Pass GIT_VERSION/DOCKER_VERSION to release env pipeline' and 'Remove Helm based release environment QA' to 18-6-stable-ee

What does this MR do and why?

Backport of the following to 18-6-stable-ee:

• 2064c257 - Pass GIT_VERSION to release environment pipeline (gitlab-com/gl-infra/production#21306 (closed))
• e9eb0666 - Pass down DOCKER_VERSION to release env pipeline (https://gitlab.com/gitlab-com/gl-infra/production/-/work_items/22168)
• 6c18faff - Remove Helm based release environment QA (gitlab-com/gl-infra/delivery#21596 (closed))
• fc68b7bc - Do not allow release environment omnibus builds to fail (gitlab-com/gl-infra/delivery#21244 (closed))

The Helm-based release environments don't work with GitLab chart for version 19.0 and above anymore. This removes the old Helm-based QA job and promotes the GET-based QA job as the primary one. GIT_VERSION and DOCKER_VERSION were missing from the inherited variables passed to the child pipeline. The Omnibus build jobs are now stable and should not be allowed to fail silently.

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 MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.