Open external links in a new tab using rel="noopener"

Before the Hugo migration, external links opened in a new tab using rel="noopener noreferrer". We had to remove noopener because Hugo didn't support it. But @grote has pointed out that it's now possible with Hugo:

Hugo 0.62 now supports Markdown Render Hooks which can be used for things like adding rel="noopener" attributes on all links.