eslint stuck at major version 8

[gwhitney]

In response to #307, I tried to go whole hog and update to the latest eslint 9.1.1. The difficulty is that eslint 9 has a whole new configuration file system called "Flat Configs". It is possible to get eslint to run using the information at: vuejs/eslint-config-typescript#76 and https://www.npmjs.com/package/eslint-config-flat-gitignore. However, our full formatting setup depends on prettier-eslint, which currently just doesn't support Flat Configs, see prettier/prettier-eslint#947. When that issue is resolved, we can try again to move eslint forward.