PHP 7.3 is EOL upstream as of December 2021 - https://www.php.net/supported-versions.php
Do we want to drop support for PHP 7.3 in MW-1.38-release or MW-1.39-release?
By the time 1.38 is released in May 2022, PHP 7.3 will be EOL for ~6 months, and for 1.39, 11 months.
Definitely wants to happen in/for MW 1.39, but the question is whether we want to get it included in MW 1.38 too.
Obviously, master will still support PHP 7.2 until T271736: Migrate WMF production from PHP 7.2 to PHP 7.4 has happened, and if we do drop PHP 7.3 in the release branch, it'll be a soft drop (same as for PHP 7.2 in current release branches).