Modernize connect-ready for Kubernetes and Node.js 22

[didiercolens]

Summary

• retain the focused readiness status API
• add a standardized, bounded HTTP shutdown lifecycle for Kubernetes
• handle SIGTERM/SIGINT as successful shutdowns and fatal process events as failures
• drain active HTTP requests, clean up dependencies, and force-close at configurable deadlines
• remove the legacy shutdown API, keepalive middleware, and toobusy support
• replace all runtime and development dependencies with native Node.js functionality
• require Node.js 22+
• document long-request and upgraded-connection shutdown behavior
• restrict the published package to runtime files

The package version remains 1.0.14 in this feature PR. After merge, npm version major will create the 2.0.0 release commit and v2.0.0 tag on master.

Breaking changes for 2.0

• replaces shutdown() with registerShutdownHandlers()
• removes gracefulShutdownKeepaliveConnections()
• removes enableTooBusy()
• validates readiness statuses as HTTP codes from 100 through 599
• requires Node.js 22+

Verification

• npm test (16 tests)
• npm run test:cover (100% function coverage)
• npm run lint
• npm pack --dry-run
• GitHub Actions CI