Update stylelint rule function-linear-gradient-no-nonstandard-direction to be consistent with Drupal's CSS standards

Problem/Motivation

Follow-up to #2865971: Use stylelint as opposed to csslint in core. Configure the function-linear-gradient-no-nonstandard-direction to be consistent with https://www.drupal.org/docs/develop/standards/css/css-coding-standards

Proposed resolution

Brief instructions on running stylelint - you'll need npm...

All the commands below take place in DRUPAL_ROOT/core
To install stylelint

npm install

This will install Drupal 8's npm dependencies of which stylelint is one.

To run it on all core css files. Apply this issue's patch and do the following command from DRUPAL_ROOT/core

npm run lint:css

Remaining tasks

User interface changes

None

API changes

None