Fix display of warning text in High Contrast Mode

When Windows High Contrast Mode (HCM) is enabled for Edge, the warning text icon is affected by the readability backplate [1] which draws a box around the exclamation mark, obscuring parts of the circle.

Opt out of the readability backplate by setting `forced-color:adjust: none` and handle it ourselves, by overriding the colors to system colours in a media query which detects if forced colors are active.

Use `forced-color-adjust` and `forced-colors` over the MS specific `-ms-high-contrast-adjust: none;` and `-ms-high-contrast: active` to allow for future support from other browsers (e.g. Firefox, which allows users to set their own colour scheme for pages).

HCM with IE11 does not seem to be affected by the same issue – I don’t think it implements a readability backplate.

[1]: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/6208eaffa0362ca107644c99b902c0b06bd5a0c8/Accessibility/HighContrast/explainer.md#ensuring-readability

src/govuk/components/warning-text/_index.scss

@@ -43,6 +43,16 @@
     // Prevent the exclamation mark from being included when the warning text
     // is copied, for example.
     user-select: none;
+
+    // Improve rendering in Windows High Contrast Mode (Edge), where a
+    // readability backplate behind the exclamation mark obscures the circle
+    forced-color-adjust: none;
+
+    @media screen and (forced-colors: active) {
+      border-color: windowText;
+      color: windowText;
+      background: transparent;
+    }
   }
 
   .govuk-warning-text__text {