Add Server.close_clients() and Server.abort_clients() - #782

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608, edgedb/action-release/validate-pr@bae6b9134e872166b43d218dd79397c851c41c9a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/