Upgrade/Install: Update sodium_compat to v1.16.1.

The latest version of sodium_compat includes polyfills for new features slated to land in PHP 8.1:
​https://paragonie.com/blog/2021/05/ristretto255-for-php-community

It also fixes a race condition with the autoloader that caused an "undefined constant" error on some systems:
​https://github.com/paragonie/sodium_compat/issues/122

A full list of changes in this update can be found on GitHub:
​https://github.com/paragonie/sodium_compat/compare/v1.14.0...v1.16.1

Follow-up to [49741].

Props paragoninitiativeenterprises, oxyrealm.
Merges [51002] to the 5.7 branch.
Fixes #53274.