Add instance setting to control use of pipeline variables

What does this MR do and why?

This change adds a new feature to disable pipeline variables in CI/CD operations. It introduces a boolean setting called pipeline_variables_disabled that administrators can toggle in the application settings.

When enabled, this setting prevents the use of variables in pipelines and will be automatically applied to any new groups created afterward. The implementation includes database migrations to add the new column to the application settings table, with a special case for GitLab.com where the setting is enabled by default.

The admin UI is updated with a new checkbox in the CI/CD settings section that allows administrators to control this feature, along with explanatory text about its effects.

References

#515794 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Go to /admin/application_settings/ci_cd and check the Variables section at the top.
2. Try toggling the setting.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #515794 (closed)