Upgrade/Install: Update sodium_compat to v1.21.2.
Version 1.21.2 is mostly to prevent deprecation warnings on PHP 8.5. These should never be encountered in practice (as the only file in scope was an `SplFixedArray` polyfill for PHP 5.2, which should never be executed on PHP 8.5), but for completeness the polyfill was updated.
References:
* [https://github.com/paragonie/sodium_compat/releases/tag/v1.21.2 sodium_compat 1.21.2 release notes]
* [https://github.com/paragonie/sodium_compat/compare/v1.20.1...v1.21.2 Full list of changes in sodium_compat 1.21.2]
Follow-up to [55699], [58752], [58753].
Props paragoninitiativeenterprises, jrf, johnbillion, TobiasBg, SergeyBiryukov.
Fixes #64008.
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