cssnano version 6

[ludofischer]

svgo 3 has been released which drops Node.js < 14 and there's a PR to update it in cssnano. We could take advantage of that to release a new major version which would include a few breaking changes besides. Here's a list

Possible changes

• upgrade to svgo 3 and drop Node < 14 cmpatibility
• remove the YAML parser dependency since YAML config are undocumented and probably unused
• possibly remove the normalize-url dependency and rewrite postcss-normalize-url to only have the current default behaviour. We can't update normalize-url since it has become ESM-only and we don't need all the options that it provides because we only want to minimize. If people don't like the result they can just turn it off rather than fiddling with options as the size savings are likely minimal. Also spec compliance is not an issue, since there is no official definition of a 'normalized' URL that I know of.

Release Tasks

• Branch version 5, master is version 6
• Remove Node.js 10 and 12 from CI on master
• Bump cssnano, cssnano-preset-default, cssnano-preset-advanced and other plugins
  (what about plugins without changes? Should they keep their old major version number? In theory they would stil be compatible with Node 10 and 12, but if we don't test those versions in CI any more should we just drop Node.js < 14 everywhere?)
• Some kind of release announcement