Enable custom Code dropdown clients by default

Stanislav Arequested to merge
stanislav43/gitlab:feature/code-dropdown-flag-default-on into master

What does this MR do and why?

Enables the custom_code_dropdown_clients feature flag by default, so administrators on GitLab Self-Managed and GitLab Dedicated can add custom "Open with" clients to the project Code dropdown list. The flag has been enabled on GitLab.com since 2026-10-02.

Also updates the docs: the history now says the flag is enabled by default, and Offering now includes GitLab Dedicated.

Related to #630394 (rollout issue)

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