Replace owner with first_owner in service specs

What does this MR do and why?

Use project.first_owner instead of project.owner for specs where we're testing something to do with a project owner, but it doesn't matter which one.

This MR only covers spec/services and there will be other MRs to do the rest in chunks (there were over 689 occurrences of .owner but some of them will be unrelated).

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

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #350603 (closed)