Allow sorting for listing milestones via GraphQL and expose expired attr [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Resolves #333354 (closed)

• Adds a new custom sort method on the Milestone model (sort_with_expired_last) and updates the Milestone finder to use the method.
• (API change) Allow querying milestones with sort options.
• (API change) Exposes expired field on Milestone GraphQL type.
• Refactors milestone related specs to run faster by using let_it_be and build

Why?

The frontend application has been shaping/re-ordering fetched milestones on the client side in this fashion:

1. Current milestones (due date > current date) are displayed on the top
2. Milestones without due dates are next
3. Expired milestones (due date <= current date) are displayed in the bottom

Milestones are then sorted by due date (asc) in each layer. This MR adds a backend support for this custom order.

More context available in this discussion thread #333354 (comment 598786176).

Sample GraphQL Queries

{
  group(fullPath: "gitlab-org") {
    milestones(sort: CREATED_ASC) {
      nodes {
      	title
        dueDate
        expired
      }
    }
  }
}

{
  group(fullPath: "gitlab-org") {
    milestones(sort: EXPIRED_LAST_DUE_DATE_ASC) {
      nodes {
      	title
        dueDate
        expired
      }
    }
  }
}

Database Review

SELECT "milestones".* 
FROM ((SELECT "milestones".* FROM "milestones" WHERE 1 = 0) 
    UNION ALL (SELECT "milestones".* FROM "milestones" WHERE "milestones"."group_id" = 9970)) milestones 
WHERE 
  ("milestones"."state" IN ('active')) 
ORDER BY 
  (CASE WHEN due_date IS NULL THEN 1 WHEN due_date > now() THEN 0 ELSE 2 END) ASC, 
  "milestones"."due_date" ASC, 
  "milestones"."id" DESC

Query Plan: https://explain.depesz.com/s/ioHB

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [-] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.