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/partials/templates/blocks.html

@@ -1,10 +1,10 @@
 {{ .ctx.Scratch.Set "sections" slice }}
 <ul id="markdown-toc">
 {{ range .ctx.Scratch.Get "blocks" }}
-{{ if .content }}
-{{ $.ctx.Scratch.Set "section" .content }}
-{{ else}}
+{{ if .block }}
 {{ $.ctx.Scratch.Set "section" (partial "templates/block" .) }}
+{{ else}}
+{{ $.ctx.Scratch.Set "section" .content }}
 {{ end }}
 {{ $section := $.ctx.Scratch.Get "section" }}
 {{ $headers := findRE "<h2.*?>(.|\n)*?</h2>" $section }}