Update stylelint rules color-hex-case to be consistent with Drupal's CSS standards

Problem/Motivation

Follow-up to #2865971: Use stylelint as opposed to csslint in core. Configure the color-hex-case to be consistent with https://www.drupal.org/docs/develop/standards/css/css-coding-standards

From the CSS Formatting Guidelines:

When hex values are used for colors, use lowercase. Colors may be expressed with any valid CSS value, such as hex value, color keyword, rgb() or rgba(). Note that IE8 does not support all color syntaxes and will require a fallback value.

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