notification_service_spec.rb: Convert some let to let_it_be to save re-creating DB records

What does this MR do?

Related issues: #241148, https://gitlab.com/gitlab-org/plan/-/issues/145

The most common usage was creating a public project, so I created a public project to be available to all spec examples.

Queries saved: 3103

Before:

Total events: 325847

After:

Total events: 322744

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.