Update all comments and documentation in Package Manager to refer to "sandboxes" and related terminology

Problem/Motivation

This is the second part of 📌 Clarify/replace 'stage' terminology in Package Manager Active . In that issue, we renamed many classes in Package Manager to use the clearer terminology proposed in #3474876-5: Rename classes that reference a "stage" to be clearer and use the term "sandbox" instead → . To keep the scope manageable, lest the MR turn into a ceaselessly mutating Tetsuo-like blob monster (see Akira if you haven't already), all documentation and comment updates have been deferred to this issue.