We originally started using symfony/process because kzykhys/pygments
depended upon it. But that library was unmaintained and became broken,
so we stopped using it, and just used symfony/process directly.

Today the only reason in favor of symfony/process is that it can pass
stdin to pygments, while Shell\Command can't (T182463). But there are
downsides, like not respecting the default MediaWiki shell limits,
cannot be used with core's firejail support, and requiring an external
composer dependency.