Replace owner with first_owner in request specs

What does this MR do and why?

We're moving towards Owner role being possible in Projects. Because multiple owners will be possible soon, we are phasing out owner.

This refactor MR replaces project.owner with project.first_owner in request specs. This assumes that "any owner" will be okay, so the first one is chosen. There should be no material changes to the behaviour of the code.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #350603 (closed)