Increasing dependencies

[weaverryan]

This is (probably) the last PR before releasing Encore 3.0 - it bumps all out-of-date dependencies (except for a few internal libraries):

• css-minimizer-webpack-plugin 3.4->4.0 (just dropping Node 12 support)
• less-loader 10->11
• postcss-loader 6->7
• sass-loader 13 (drops support for 12)
• stylus 0.57->0.58
• styus-loader 6->7

Cheers!

[stof]

@weaverryan any reason to lock assets-webpack-plugin to 7.0.* instead of using the 7.1 version ?

[weaverryan]

Yes, fair question - it's this: #969

I haven't looked into it since then to see what we can do.