Flip value of selector-pseudo-element-colon-notation

The Design System team is in the process of removing support for Internet Explorer 8, 9 and 10 from GOV.UK Frontend, signalling our intent to no longer support the browser or provide fixes or polyfills specifically for it. 

Supporting IE8–10 has not been a requirement for services since June 2018.

Barring legacy projects, we do not expect services to be making accommodations for legacy IE versions now or in future, and encourage those projects to begin removing any code specifically for these versions of IE.

As part of this, we should update the value of `selector-pseudo-element-colon-notation` to begin enforcing the standards-compliant double-colon syntax instead.

css-rules.js

@@ -151,12 +151,9 @@ module.exports = {
     // compatibility in older browsers
     // https://stylelint.io/user-guide/rules/selector-not-notation/
     'selector-not-notation': 'simple',
-    // Require single colons for defining pseudo-elements
-    // IE8 and below do not support the modern double colon approach. Although
-    // few projects support IE8, we'd prefer to not exclude compatibility for
-    // purely syntactic reasons.
+    // Require double colons for defining pseudo-elements
     // https://stylelint.io/user-guide/rules/list/selector-pseudo-element-colon-notation/
-    'selector-pseudo-element-colon-notation': 'single',
+    'selector-pseudo-element-colon-notation': 'double',
     // Disallow redundant properties in rules (for example: margin: 1px 1px 1px;)
     // https://stylelint.io/user-guide/rules/list/shorthand-property-no-redundant-values/
     // Originates from: https://github.com/alphagov/govuk-frontend/blob/e248b4027102b2684f592a0501630075bdfa1fab/config/.sass-lint.yml#L436