Front page missing hreflang links

Problem/Motivation

If you're using separate front pages, they don't have hreflang links indicating alternate languages are available (i.e. <link rel="alternate" hreflang="de" href="https://example.com/" />), which is important for SEO.

Steps to reproduce

1. View source on a site with a normal translated front page.
2. Observe that it has <link rel="alternate" hreflang"[langcode]" href="https://example.com/" /> for each language.
3. Set up separate front pages per language.
4. View source and observe each front page only has an hreflang link to itself.

Proposed resolution

Implement hook_page_attachments() to add the hreflang links.

User interface changes

None.