Update to css-loader 3.6.0

[pjonsson]

What this PR does

There seems to be some cross-dependency
interplay between css-loader, style-loader,
and mini-css-extract-plugin.

Update css-loader by one major version,
which seems to be the most recent version
that works with the current versions
of the other two libraries.

Test me

I believe this is tested by CI, bumping to the next major gave lots of empty scss files and then failed.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.