Update Webpack to next major version (webpack 5)

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We eventually want to update to webpack 5

Loaders and other webpack related dependencies

Below (a currently non-exhaustive list) of all loaders and their status quo,

Compatible	Name	Current Version	Minimum version needed for webpack 5	caveats
✅	compression-webpack-plugin	5.0.2	3.1.0	-
!60061 (merged)	copy-webpack-plugin	5.1.2	6.0.4	-

Config changes

• We should double-check the publicPath config. We currently set it in JS land based on Gon. It seems like there is an auto option in webpack 5: https://webpack.js.org/configuration/output/#outputpublicpath / https://webpack.js.org/guides/public-path/#automatic-publicpath. We would need to check whether this works with all loaders. It would enable us to potentially remove: https://gitlab.com/gitlab-org/gitlab/-/blob/034d36ae/app/assets/javascripts/webpack.js