Fix: roadmap page height calculation issue

What does this MR do and why?

Related to issue #3081 (closed), this MR adds the global footer height to the roadmap canvas height calculation, so that the height of the canvas can be correctly drawn.

This MR will only be effective after https://gitlab.com/gitlab-org/gitlab/-/merge_requests/123344 gets merged.

Fix: roadmap page height calculation issue

Add the footer height to the calculation method.

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