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/case-studies/list.html

@@ -48,7 +48,7 @@
 			{{ $logo := .Resources.GetMatch "**logo*.png" }}
 			{{ $p := . }}
 			<a target="_blank" href="{{ with $p.Params.content_url }}{{ . | safeURL }}{{ else }}{{ .RelPermalink }}{{ end }}">{{ with $logo }}<img src="{{ .RelPermalink }}" alt="{{ $p.LinkTitle }}">{{ else }}
-				{{ errorf "Case Studies: Missing logo for %s. Put a PNG with the word 'logo' in the filename into %q" $p.LinkTitle $p.Dir }}
+				{{ errorf "Case Studies: Missing logo for %s. Put a PNG with the word 'logo' in the filename into %q" $p.LinkTitle $p.File.Dir }}
 			{{ end }}</a>
 			{{ end }}
 			<a target="_blank" href="https://docs.google.com/a/google.com/forms/d/e/1FAIpQLScuI7Ye3VQHQTwBASrgkjQDSS5TP0g3AXfFhwSM9YpHgxRKFA/viewform" class="tell-your-story"><img src="/images/case_studies/story.png" alt="Tell your story"></a>