refactor(ssr): update g6-ssr

[Aarebecca]

• 调整 Graph 接口，如下：

export interface Graph {
  getGraph: () => G6Graph;
  getCanvas: () => Canvas;
  destroy: () => void;

  exportToFile: (file: string, meta?: MetaData) => void;
  toBuffer: (meta?: MetaData) => Buffer;
  toDataURL: () => string;
}

• 更新测试用例及截图