Run liquid-ifversion-tags in content frontmatter too (#48990)

Co-authored-by: Rachael Sewell <rachmari@github.com>

src/content-linter/style/github-docs.js

@@ -142,6 +142,11 @@ export const githubDocsFrontmatterConfig = {
     severity: 'error',
     'partial-markdown-files': false,
   },
+  'liquid-ifversion-tags': {
+    // GHD020
+    severity: 'error',
+    'partial-markdown-files': false,
+  },
 }
 
 // Configures rules from the `github/markdownlint-github` repo