Refactor Organization scope for getting Organizations by users

What does this MR do and why?

This is a small refactor that will allow for more flexibility for retrieving organizations that belong to a certain user.

Follow-up from !158626 (comment 2010176114)

Query Plans

• Scope Organizations::Organization.with_user(user)
  • Master branch: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/30197/commands/93638
  • This branch: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/30197/commands/93636
  • Query is more efficient because the join on users table is now removed
• Scope Organizations::OrganizationUser.by_user
  • Master branch: N/A
  • This branch: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/30197/commands/93637

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #443829 (closed)