Resolve "Replace '.team << [user, role]' with 'add_role(user)' in specs"

What does this MR do?

1. Removes app/models/project_team.rb#<<.
2. Adds app/models/project_team.rb#add_role.
3. Removes project.team << [user, role] methods with project.add_*(user) or project.add_role(user, role) both in spec/ and in features/.

Why was this MR needed?

Make specs consistent and human readable.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #36782 (closed), !9232 (closed)