Dependency major version updates

[fhennig]

There are a lot of major version bumps which require new node versions, breaking changes etc.
This needs to be tackled manually.

Package                    Current   Wanted   Latest  Location                                Depended by
cssnano                     4.1.11   4.1.11   5.1.14  node_modules/cssnano                    documentation-ui
eslint                       6.8.0    6.8.0   8.31.0  node_modules/eslint                     documentation-ui
eslint-config-standard      14.1.1   14.1.1   17.0.0  node_modules/eslint-config-standard     documentation-ui
eslint-plugin-promise        4.3.1    4.3.1    6.1.1  node_modules/eslint-plugin-promise      documentation-ui
eslint-plugin-standard       4.1.0    4.1.0    5.0.0  node_modules/eslint-plugin-standard     documentation-ui
fancy-log                    1.3.3    1.3.3    2.0.0  node_modules/fancy-log                  documentation-ui
fs-extra                     9.1.0    9.1.0   11.1.0  node_modules/fs-extra                   documentation-ui
gulp-imagemin                7.1.0    7.1.0    8.0.0  node_modules/gulp-imagemin              documentation-ui
highlight.js                10.7.3   10.7.3   11.7.0  node_modules/highlight.js               documentation-ui
postcss-custom-properties   11.0.0   11.0.0  12.1.11  node_modules/postcss-custom-properties  documentation-ui
postcss-import              14.1.0   14.1.0   15.1.0  node_modules/postcss-import             documentation-ui
prettier-eslint             12.0.0   12.0.0   15.0.1  node_modules/prettier-eslint            documentation-ui
stylelint                  13.13.1  13.13.1  14.16.1  node_modules/stylelint                  documentation-ui
stylelint-config-standard   22.0.0   22.0.0   29.0.0  node_modules/stylelint-config-standard  documentation-ui

Edit: Since the project is built with gulp, it looks like you need to uprade the gulp dependencies. Some of them don't seem to receive updates anymore. It's all a bit messy. For example eslint is used by gulp-eslint, but the package is abandoned. In general, Gulp seems to have lost a lot of steam.

[fhennig]

It would be good to investigate more how to build process actually works #30