chore: bump minimum nodejs version to 8

[legobeat]

Bumps minimum nodejs version from v6 to v8.

This is to reflect the current state of things as nodejs v6 throws the following on npm install:

npm ERR! Linux 6.2.10-1.qubes.fc32.x86_64
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "i" npm ERR! node v6.17.1
npm ERR! npm  v3.10.10
npm ERR! path /app/node_modules/.staging/@types/minimatch-fad7cd5e/package.json npm ERR! code ENOTDIR
npm ERR! errno -20
npm ERR! syscall open

npm ERR! ENOTDIR: not a directory, open '/app/node_modules/.staging/@types/minimatch-fad7cd5e/package.json' npm ERR!
npm ERR! If you need help, you may report this error at:
npm ERR!     <https://github.com/npm/npm/issues>

npm ERR! Please include the following file with any support request:
npm ERR!     /app/npm-debug.log

Also updates the CI Matrix accordingly, and adds more recent maintenance releases v16 and v18.