Fix deprecated string interpolation syntax

[mrhead]

The old syntax is deprecated since PHP 8.2.

See:

• https://php.watch/versions/8.2/%24%7Bvar%7D-string-interpolation-deprecated
• https://www.php.net/manual/en/language.types.string.php#language.types.string.parsing

Fixes: https://3.basecamp.com/3293071/buckets/5610905/card_tables/cards/7252740605