CI: Bump Node version to latest LTS

[pete-murphy]

Seeing the following error in the CI build for #304

=== Building HelloLog for the web browser backend ===
/home/runner/work/purescript-cookbook/purescript-cookbook/node_modules/@parcel/utils/lib/index.js:37573
    #getIndex(item) {
             ^

SyntaxError: Unexpected token '('

#getIndex is using relatively new private class field syntax. According to MDN, that should be valid in Node v12 so I'm not sure why this is failing with a syntax error. Trying to bump to v18 to see if that helps.

[pete-murphy]

I'll go ahead and merge this, just a version bump that appears to fix CI following a breaking change in parcel