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/capture.html

@@ -1,3 +1,4 @@
+{{ $_hugo_config := `{ "version": 1 }`}}
 {{- $id := .Get 0 -}}
 {{- if not $id -}}
 {{- errorf "missing id in capture" -}}