Remove node 12 ci build-test

[birkskyum]

I'd recommend removing this node 12 test for the following reasons:

• newer versions of i.e. Jasmine, node-polyfill-webpack-plugin and others have sunset node 12, keeping this repo with deprecated deps like querystring@0.2.0 in perpetuity. It's effectively impossible to move the tooling of this repo forward with this limitation.
• it's effectively only affecting maintainers, since users won't have to make a build of plotly, but just consume one, which is dependant only on the compile output rather which runtime is used for development of plotly.js
• usage of node 12 should be discourage due to security reasons, having no active nor security support for a long time.

[archmoj]

Please add draftlogs/7095_change.md file.
Thanks for the PR.

[archmoj]

💃