Shortcodes: drop support for pre-Hugo-0.54.x behavior of `{{% %}}`

[chalin]

Here is some context:

• #87

While the change introduced by that PR was relevant at the time (during a transition period from Hugo 0.54.x to 0.55+), we now only officially support the latest version of Hugo. So, all shortcodes making use of

{{ $_hugo_config := `{ "version": 1 }` }}

should have that line removed, and the shortcode adjusted, if necessary.

$ find . -type f -not -path "*/node_modules/*" -exec grep -le '_hugo_config' {} \;
./layouts/shortcodes/alert.html
./layouts/shortcodes/blocks/cover.html
./layouts/shortcodes/blocks/section.html
./layouts/shortcodes/blocks/lead.html
./layouts/shortcodes/pageinfo.html

• [x] layouts/shortcodes/alert.html - #941
• [ ] layouts/shortcodes/blocks/cover.html - #1400
• [x] layouts/shortcodes/blocks/section.html - #1472
• [x] layouts/shortcodes/blocks/lead.html - #1398
• [ ] layouts/shortcodes/pageinfo.html