Change fullstack engineering manager to frontend and backend

Why is this change being made?

When you check https://about.gitlab.com/handbook/product/categories/, you can see that all the EM's that were listed as fullstack_engineering_manager in stages.yml (in !120419 (merged)) do not show up. It doesn't seem like we have a keyword for that, as the other fullstack EMs are just listed twice, for backend/frontend. As a quick fix, I think we could just do the same for the updates from that MR?

Related to !120419 (comment 1311012269)

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