Add name argument to workItemTypes field

What does this MR do and why?

Replace taskable argument with name

Add name argument to workItemTypes GQL field to be able to query a specific type.

Also, remove the existing (experimental) taskable argument because it becomes redundant.

Example query

query {
  workspace: group(fullPath: "group-a") {
    workItemTypes(name: TASK) {
      edges {
        node {
          id
          name
          iconName
          widgetDefinitions {
            type
          }
        }
      }
    }
  }
}

example response

{
  "data": {
    "workspace": {
      "workItemTypes": {
        "edges": [
          {
            "node": {
              "id": "gid://gitlab/WorkItems::Type/5",
              "name": "Task",
              "iconName": "issue-type-task",
              "widgetDefinitions": [
                {
                  "type": "ASSIGNEES"
                },
                {
                  "type": "LABELS"
                },
                {
                  "type": "DESCRIPTION"
                },
                {
                  "type": "HIERARCHY"
                },
                {
                  "type": "START_AND_DUE_DATE"
                },
                {
                  "type": "MILESTONE"
                },
                {
                  "type": "NOTES"
                },
                {
                  "type": "ITERATION"
                },
                {
                  "type": "WEIGHT"
                },
                {
                  "type": "NOTIFICATIONS"
                },
                {
                  "type": "CURRENT_USER_TODOS"
                },
                {
                  "type": "AWARD_EMOJI"
                },
                {
                  "type": "LINKED_ITEMS"
                }
              ]
            }
          }
        ]
      }
    }
  }
}

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