Add filtering option to `vulnerabilityGrades` field

What does this MR do and why?

This MR introduces letterGrade argument for the Group#vulnerabilityGrades, and InstanceSecurityDashboard#vulnerabilityGrades fields.

Related to Improve the `group#vulnerabilityGrades` GraphQL... (#350110 - closed), and Improve the `instanceSecurityDashboard#vulnerab... (#350622 - closed).

Testing locally

Assuming you already have a group and vulnerable projects on your development environment, the following queries can be used to test this functionality;

For the group-level field

query getGradesUnderGroup {
  group(fullPath: "group path") {
    vulnerabilityGrades(includeSubgroups: true, letterGrade: F) {
      grade
      count
      projects {
        pageInfo {
          endCursor
          hasNextPage
        }
        nodes {
          name
        }
      }
    }
  }
}

For the instance-level field

query getGradesForInstance {
  instanceSecurityDashboard {
    vulnerabilityGrades(letterGrade: F) {
      grade
      count
      projects {
        nodes {
          name
        }
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.