Exclude code owner from merge request participants to avoid spam

What does this MR do?

Code owner was included as part of approvers. However this would cause notifications and todos, which can spam code owners.

Prevent notifications/todos when new code owners becomes approvers.

• RefreshService: this detects new code owners and sets them as approvers, now its notification creation is removed.
• UpdateService: it finds difference in approver and sends out notifications to new approvers. Now code owners are excluded from this.
• TodoService: Exclude code owners for todo creation3
• MergeRequest#participant_approvers: Now exclude code owners

What are the relevant issue numbers?

Closes #8396 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?