support node16

[tmatsuo]

This repo doesn't support node16, but we started to have a presubmit build with node16.

npm i failed with node16:
build

running test in sessions
/workspace/sessions /workspace
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! 
npm ERR! While resolving: session-handling@undefined
npm ERR! Found: @google-cloud/firestore@5.0.2
npm ERR! node_modules/@google-cloud/firestore
npm ERR!   @google-cloud/firestore@"^5.0.0" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peer @google-cloud/firestore@"^4.0.0" from @google-cloud/connect-firestore@2.0.2
npm ERR! node_modules/@google-cloud/connect-firestore
npm ERR!   @google-cloud/connect-firestore@"^2.0.0" from the root project
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR! 
npm ERR! See /builder/home/.npm/eresolve-report.txt for a full report.

[tmatsuo]

wontfix