Fix 'wcag/h71' violations

Some of our examples have a fieldset with no legend, which violates this rule:

> <fieldset> must have a <legend> as the first child [wcag/h71]

Add a legend to all examples that use a fieldset and are currently missing a legend.

src/govuk/components/all.test.js

@@ -103,9 +103,7 @@ describe.each(allComponents)('%s', (component) => {
           //
           // Relax the rule to allow anything that is valid according to the
           // spec.
-          'valid-id': ['error', { relaxed: true }],
-
-          'wcag/h71': 'off'
+          'valid-id': ['error', { relaxed: true }]
         },
         elements: [
           'html5',

src/govuk/components/checkboxes/checkboxes.yaml

@@ -699,6 +699,8 @@ examples:
     name: example-name
     fieldset:
       describedBy: some-id
+      legend:
+        text: Which option?
     items:
       - value: 1
         text: Option 1

src/govuk/components/fieldset/fieldset.yaml

@@ -114,6 +114,8 @@ examples:
   hidden: true
   data:
     describedBy: some-id
+    legend:
+      text: Which option?
 - name: html as text
   hidden: true
   data:
@@ -134,12 +136,18 @@ examples:
   hidden: true
   data:
     classes: app-fieldset--custom-modifier
+    legend:
+      text: Which option?
 - name: role
   hidden: true
   data:
     role: group
+    legend:
+      text: Which option?
 - name: attributes
   hidden: true
   data:
     attributes:
       data-attribute: value
+    legend:
+      text: Which option?

src/govuk/components/radios/radios.yaml

@@ -692,6 +692,8 @@ examples:
     name: example-name
     fieldset:
       describedBy: some-id
+      legend:
+        text: Which option?
     items:
       - value: yes
         text: Yes