Re-generate Coderay syntax highlighting styles [OP 3.x backport]

Coderay 1.0+ now uses more descriptive class names.
We use the default (alpha) styles:

`coderay stylesheet > app/assets/stylesheets/coderay.css`

https://community.openproject.org/work_packages/17114
@hschink You're quite right. I've amended the PR. I made changes to use Sprockets directives rather than `@import` in a couple of places, but forgot that these commits only ended up on 4.x branch.