deps: update dependency com.google.cloud:gapic-libraries-bom to v1.91.1 - #2000

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: abcxyz/actions/.github/actions/multi-approvers@893209ed79a3d2508eeec375ffaf3d21012f5cd0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/