Add category create and update mutations

What does this MR do and why?

Adds Security Category create and update mutations

Changelog: added
EE: true

Related to Add security categories mutations (#562233 - closed) • Gal Katz • 18.4

How to set up and validate locally

1. Enable security_categories feature flag:
   Feature.enable(: security_categories_and_attributes)
2. Use this GraphQL to create categories:

    mutation verify {
      securityCategoryCreate(input: {
        namespaceId: "gid://gitlab/Group/<GROUP_ID>",
        name:"This is a test",
        description:"This is a desc",
        multipleSelection: true,
      }){
        securityCategory {
          id
          name
          description
        },
        errors
      }
    }

3. Use this GraphQL to update categories:

    mutation verify {
      securityCategoryUpdate(input: {
        id: "gid://gitlab/Security::Category/<CATEGORY_ID>",
        namespaceId: "gid://gitlab/Group/<GROUP_ID>",
        name: "New name",
        description: "New description"
      }){
        securityCategory{
          id
          name
          description
        },
        errors
      }
    }

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