Explicitely set Warning Text font weight

There's no guarantee browser defaults of `bold` or `bolder` have not been overriden
so we need to set the font-weight explicitely.

Closes #5279

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

@@ -8,7 +8,7 @@
 
   .govuk-warning-text__icon {
     // We apply this here and not at the parent level because the actual text is
-    // a <strong> and so will always be bold
+    // wrapped in a `<strong>` which will not inherit the value
     @include govuk-typography-weight-bold;
     box-sizing: border-box;
 
@@ -55,6 +55,9 @@
 
   .govuk-warning-text__text {
     @include govuk-text-colour;
+    // While `<strong>` is styled `bold` or `bolder` by user-agents
+    // this can be reset by the app's stylesheet so we need to set it ourselves
+    @include govuk-typography-weight-bold;
     display: block;
     padding-left: 45px;
   }