Fix userNotesCount aggregate

What does this MR do?

I've noticed that, when we combine Vulnerability.userNotesCount with vulnerabilityGrades in single query we receive no data, invalid data or data is not returned because of the error. This is because we were using incorrectly same key in query context for 2 different aggregates instead of using the one that is dedicated to given aggregate (https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/graphql/ee/gitlab_schema.rb#L11). This MR resolves that problem.

GraphQL Query Examples

{
  group1: group(fullPath: "example1") {
    vulnerabilityGrades {
      grade
      count
      projects {
        nodes {
          vulnerabilities {
            nodes {
              id
              userNotesCount
            }
          }
        }
      }
    }
  }
  group2: group(fullPath: "example2") {
    vulnerabilityGrades {
      grade
      count
      projects {
        nodes {
          confirmedVulnerabilities: vulnerabilities(state: CONFIRMED) {
            nodes {
              id
              userNotesCount
            }
          }
          dismissedVulnerabilities: vulnerabilities(state: DISMISSED) {
            nodes {
              id
              userNotesCount
            }
          }
        }
      }
    }
  }
}

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.
• [-] 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