Owners of personal projects to have OWNER access level

What does this MR do and why?

For personal projects, the owner is added with MAINTAINER access level as a member. However, they are granted OWNER level permissions. This is because we have a workaround to disregard their access level, and only check if they are the Project#owner.

We can simplify this behaviour and have Project#owner as a member with OWNER access level.

Related to #350604 (closed) and #219299 (closed).

Screenshots or screen recordings

The current behaviour adds the owner as MAINTAINER as a project member:

With the changes in this MR, the owner gets added as OWNER:

How to set up and validate locally

Enable the personal_project_owner_with_owner_access feature flag:

Feature.enable(:personal_project_owner_with_owner_access)

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.