[UX] Improve UI copy in the visibility & access section

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Problem

1. When configuring an agent we use Private/Public terminology. This can be misinterpreted by users since the setting is more about visibility than permissions.

2. The agent configuration must be assigned/owned by a single project. The current Source project label and help text combo don't fully explain the impact of this choice.

Examples

Background

• Depending on configuration, only owners, maintainers, or developers in the source project can edit/delete the agent.
• The project is auto-enabled in the source project at time of creation.
• Agents visibility configuration impacts where they can be seen and run. If set to Project members (formerly Private) then it can only be viewed and run inside the source project. If set to All users (formerly Public) then it can be viewed, and run if also enabled, in consumer projects. See: https://gitlab.com/gitlab-org/gitlab/-/issues/578131#note_2843563994

Proposed solution

1. Consider renaming Source project to Owner project or Managed by
2. Reword the help text to clearly explain the difference between the owning project and a "connected" consumer project. Also how this settings determines who can edit/delete the agent.
3. Reword Private / Public to Project members / All members
4. Consider re-ordering Managed by input so it appears above the Visibility radio options since the Project members help text refers to the managing project.
5. Update the alert text to reflect the new wording. This might also need a followup issue as it's unclear why we need to show the A public agent can be made private only if it is not used. when editing an agent.