Merge pipeline maintainer into tooling maintainer

Why is this change being made?

This is a follow up from !114700 (merged) where we introduced a pipeline maintainer, but thinking about it now, it's too complex to have both tooling and pipeline maintainers.

Bottom line, I think we should only have one kind.

While I think tooling maintainer is less clear about the pipeline, and I probably prefer to call it pipeline maintainer, tooling reviewer/maintainer is already established and we have labels and Danger category around that. It's just simpler to go with tooling for now.

I might want to propose we rename tooling to pipeline at some point, but probably not something I want to persuade right now.

Next step is adding a new group for it so we can use it for CODEOWNERS: gitlab-org/gitlab!103591 (merged)

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