Bump CNG helm chart ref to v10.5.0

What does this MR do and why?

Bumps the helm chart pin used by the CNG end-to-end jobs to chart tag v10.5.0. The old pin makes e2e:test-on-cng fail on this branch (see !262739 (merged), job 17225404285). The first failure was in !262431 (merged). The chart fix is gitlab-org/charts/gitlab!5472 (merged).

There is no master MR to backport. Master builds from GITLAB_CNG_MIRROR_REF on Go 1.26.7, so it isn't affected.

MR acceptance checklist

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch. N/A: CI-only fix, master is unaffected.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com. N/A.
• The MR title is descriptive.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (N/A)
  • customer label (N/A)
• This MR has been approved by a maintainer (only one approval is required).
• The E2E Omnibus GitLab EE pipeline (e2e:test-on-omnibus-ee) has succeeded. Not run yet. This CI-config-only change doesn't touch Omnibus; the MR's e2e:test-on-cng pipeline is the real test.

Agent context

• One-line change: GITLAB_HELM_CHART_REF in .gitlab/ci/qa-common/variables.gitlab-ci.yml, from 2d1fbe943b35 to 7ba70f28945f473d1ffaafb9695046a304bca7a1 (v10.5.0).
• No other pin of the old sha on the branch. The helm cache key, the CNG jobs and the caproni job all read the variable.
• Not changed: the CNG mirror ref.
• Checked: YAML parses. Pre-push hooks passed, except mermaidlint and openapi_docs, which I skipped because they need node modules and a database config. Neither touches CI YAML.