Add IE11 support forced color mode tweaks

Adds IE11 supportto the previous Windows High Contrast Mode fixes in PR
#2264.

Internet Explorer 11 doesn't support the `forced-colors` media feature
or the `forced-color-adjust` property, but it has equivalents in
`-ms-high-contrast` and `-ms-high-contrast-adjust`.
We can use those CSS features to extend fixes to high contrast mode that
previously only worked on more modern browsers.

Adds a tweak to changes in commit 7dcba7d, so that the highlight works
in Internet Explorer 11 as well.

src/govuk/components/checkboxes/_index.scss

@@ -132,7 +132,7 @@
 
     // When in an explicit forced-color mode, we can use the Highlight system
     // color for the outline to better match focus states of native controls
-    @media screen and (forced-colors: active) {
+    @media screen and (forced-colors: active), (-ms-high-contrast: active) {
       outline-color: Highlight;
     }
 

src/govuk/components/radios/_index.scss

@@ -130,7 +130,7 @@
 
     // When in an explicit forced-color mode, we can use the Highlight system
     // color for the outline to better match focus states of native controls
-    @media screen and (forced-colors: active) {
+    @media screen and (forced-colors: active), (-ms-high-contrast: active) {
       outline-color: Highlight;
     }