Add negative filters for merge requests API

What does this MR do?

Adds negative filtering options for labels and milestone for merge requests in GraphQL. See screenshots for example

Screenshots (strongly suggested)

Database queries

Positive query

SELECT "merge_requests".*
FROM "merge_requests"
WHERE "merge_requests"."target_project_id" = 278964
  AND (EXISTS
         (SELECT "label_links".*
          FROM "label_links"
          WHERE "label_links"."target_type" = 'MergeRequest'
            AND "label_links"."target_id" = "merge_requests"."id"
            AND "label_links"."label_id" = 1
          LIMIT 1))
  AND (EXISTS
         (SELECT "label_links".*
          FROM "label_links"
          WHERE "label_links"."target_type" = 'MergeRequest'
            AND "label_links"."target_id" = "merge_requests"."id"
            AND "label_links"."label_id" = 80
          LIMIT 1))
ORDER BY "merge_requests"."created_at" DESC,
         "merge_requests"."id" DESC
LIMIT 100;

Plan: https://explain.depesz.com/s/Yl2o

Negative query

SELECT "merge_requests".*
FROM "merge_requests"
WHERE "merge_requests"."target_project_id" = 278964
  AND NOT (EXISTS
             (SELECT TRUE
              FROM "label_links"
              INNER JOIN "labels" ON "labels"."id" = "label_links"."label_id"
              WHERE "label_links"."target_type" = 'MergeRequest'
                AND "label_links"."target_id" = "merge_requests"."id"
                AND "labels"."title" IN ('HHR',
                                         'Trinix')))
ORDER BY "merge_requests"."created_at" DESC,
         "merge_requests"."id" DESC
LIMIT 100;

Plan: https://explain.depesz.com/s/Yl2o

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team