Turn on media-feature-name-no-unknown stylelint rule

This rule doesn't allow media features that aren't defined in CSS
specifications (while making an exception for vendor prefixed ones).

min-device-pixel-ratio never became an actual standard and the
min-resolution rule is preferred. min--moz-device-pixel-ratio was a
Mozilla specific rule that was dropped after Firefox 16 which is quite
far out of the browser support range this project has.

Orginates from: https://github.com/stylelint/stylelint-config-recommended/blob/59523c88fb1d83d922a3982c2ccf3c3c3b2776b3/index.js#L22

.stylelintrc.yml

@@ -4,7 +4,6 @@ ignoreFiles: src/govuk/vendor/**/*
 rules:
   declaration-no-important: ~
   indentation: ~
-  media-feature-name-no-unknown: ~
   media-feature-parentheses-space-inside: ~
   no-duplicate-selectors: ~
   no-eol-whitespace: ~

src/govuk/helpers/_device-pixels.scss

@@ -29,9 +29,7 @@
 
 @mixin govuk-device-pixel-ratio($ratio: 2) {
   @media only screen and (-webkit-min-device-pixel-ratio: $ratio),
-    only screen and (min--moz-device-pixel-ratio: $ratio),
     only screen and (  -o-min-device-pixel-ratio: #{($ratio*10)} / 10),
-    only screen and (     min-device-pixel-ratio: $ratio),
     only screen and (             min-resolution: #{($ratio*96)}dpi),
     only screen and (             min-resolution: #{$ratio}dppx) {
       @content;