Use namespace avatar instead of owner

What does this MR do and why?

As we phase out owner method in preparation for membership/accessed based Owner role in Project, this modifies one usage of owner in the codebase.

There should be no material difference since project.owner avatar will be identical.

Screenshots or screen recordings

(On my local, the top is a user namespace with an avatar set and the bottom is a group with no avatar)

How to set up and validate locally

1. Go to http://gdk.test:3000/some_group/some_project/-/settings/ci_cd and expand the Pipeline Subscriptions pane.
2. Add a subscription to another project and save
3. See the avatar is unchanged (this should be identical to the master branch).

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)