Replace govuk-tag--inactive with govuk-tag--grey in tests

Class `govuk-tag--inactive` was replaced by `govuk-tag--grey` and
deprecated in PR #1711. This commit replaces references to it in our
tests, examples, and fixtures.

src/govuk/components/phase-banner/__snapshots__/template.test.js.snap

@@ -1,7 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`Phase banner with dependant components renders the tag component classes 1`] = `
-<strong class="govuk-tag govuk-phase-banner__content__tag govuk-tag--inactive">
+<strong class="govuk-tag govuk-phase-banner__content__tag govuk-tag--grey">
   alpha
 </strong>
 `;

src/govuk/components/phase-banner/phase-banner.yaml

@@ -60,6 +60,6 @@ examples:
     data:
       text: This is a new service – your feedback will help us to improve it
       tag:
-        classes: govuk-tag--inactive
+        classes: govuk-tag--grey
         text: alpha
 

src/govuk/components/tag/tag.yaml

@@ -23,7 +23,7 @@ examples:
 - name: inactive
   data:
     text: alpha
-    classes: govuk-tag--inactive
+    classes: govuk-tag--grey
 - name: grey
   data:
     text: Grey

src/govuk/components/tag/template.test.js

@@ -30,7 +30,7 @@ describe('Tag', () => {
       const $ = render('tag', examples.inactive)
 
       const $component = $('.govuk-tag')
-      expect($component.hasClass('govuk-tag--inactive')).toBeTruthy()
+      expect($component.hasClass('govuk-tag--grey')).toBeTruthy()
     })
   })