Record first_contribution for all contributors

What does this MR do and why?

Follow on from !116844 (merged)

This removes the check for project membership when calculating first_contribution. Just because somebody is a member of a project, doesn't mean we shouldn't track their contributions!

See also https://gitlab.com/gitlab-org/community-relations/contributor-success/team-task/-/issues/205#note_1343532037 for some discussion on this

Relates to gitlab-org/community-relations/contributor-success/team-task#205

Screenshots or screen recordings

How to set up and validate locally

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.