For Node 12:

• Node 10.x is no longer officially supported: https://endoflife.date/nodejs (End of Life was April 30th, 2021)
• CI coverage is already only 12/14/16:

  mongo-express/.github/workflows/standard-ci.yml

  Line 18 in ca26053

  node: [12, 14, 16]

For Webpack:

• assets-webpack-plugin major bump that requires Webpack 5+, which we have: https://github.com/ztoben/assets-webpack-plugin/blob/master/changelog.md#700---2020-12-11
• clean-webpack-plugin major bump adds support for Webpack 5+ (we probably got away with it because we weren't using certain options) and drops Node 8: https://github.com/johnagan/clean-webpack-plugin/releases/tag/v4.0.0
• copy-webpack-plugin major bumps require Webpack 5+ (https://github.com/webpack-contrib/copy-webpack-plugin/releases/tag/v7.0.0), remove the need for . before [ext] (https://github.com/webpack-contrib/copy-webpack-plugin/releases/tag/v8.0.0) and require Node 12+ (https://github.com/webpack-contrib/copy-webpack-plugin/releases/tag/v9.0.0)
• Ran build and build-dev scripts

Closes #795