Make it compatible with Hugo 0.55 (#13680)

This commit adds two backwards compatible changes to prepare for the next Hugo version, most notable:

* All the `File` methods on `Page` has been deprecated and will give a warning in the next Hugo version. Adjust this by changing `.Dir` to ´.File.Dir` and similar.
* Version the capture shortcode to use the old behaviour for markdown rendering of shortcode inner content.

layouts/shortcodes/tabs.html

@@ -27,7 +27,7 @@
 				{{ end }}
 			{{ end }}
 		{{ else}}
-		{{ $path := path.Join $.Page.Dir .include }}
+		{{ $path := path.Join $.Page.File.Dir .include }}
 		{{ $page := site.GetPage "page" $path }}
 		{{ with $page }}
 			{{ .Content }}