[GitLab Dedicated] ✨ Link to correct docs on feature flags when describing features that are not available

What does this MR do and why?

[GitLab Dedicated] ✨ Link to docs on feature flags for administration

This MR adjusts the GitLab Dedicated subscriptions page so that the section on GitLab application features that are not available points to:

• Enable and disable GitLab features deployed behind feature flags

instead of to:

• Feature flags

I propose linking to ee/administration/feature_flags.html instead of to ee/operations/feature_flags.html.

That will help clarify what isn't available in GitLab Dedicated.

We discussed this possible confusion briefly during gitlab-com/support/support-pairing#11067 and I observed some room to clarify this in a GitLab Dedicated-related Request for Help issue.

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Before	After

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