Add allowed_download_modes Helm configuration

What does this MR do?

Exposes allowed_download_modes through Helm values, renders it into gitlab.yml, and documents the setting.

Related Rails change: gitlab-org/gitlab!252350 (merged) Companion Omnibus MR: gitlab-org/omnibus-gitlab!9748 (closed)

Related issues

Related to gitlab-org/gitlab#624081 (closed)

Author checklist

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated.
• Have you validated that your change works end-to-end on a real cluster deployment? Helm and Bundler are unavailable in this environment. Ruby syntax, YAML parsing, editor diagnostics, and git diff --check pass.
• Equivalent MR for omnibus-gitlab opened: gitlab-org/omnibus-gitlab!9748 (closed).

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• No Operator impact is expected because this change only serializes a new Rails configuration setting.