Resolve "Does not set `projects_limit` to 10 when creating user via API"

What does this MR do?

• Removes default value from users.projects_limit
• Adds NOT NULL constraint to users.projects_limit
• Simplifies the set_projects_limit after_initialize callback in the User model

Are there points in the code the reviewer needs to double check?

I added the original issue as a test in user_spec, but I'm not sure if it should be added at all (line 819)

Why was this MR needed?

See #31995 (closed)

Without these changes, a new User cannot be created with a projects_limit value that matches the DB default because it will be overridden with the current_application_settings.default_projects_limit because ActiveRecord's *_changed? methods return false if the value being set matches the DB default.

Screenshots (if relevant)

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 #31995 (closed)