4.x: Bootstrap

[akarnokd]

• Branch 3.x into 4.x <- this is on @akarnokd, depends on Create entropy-beauty-scan.yml #7966
• Refactor io.reactivex.rxjava3 to io.reactivex.rxjava4 ⚠️ lots of files, don't change anything else.
• Update mentions of RxJava 3 to RxJava 4 in many docs. ℹ️ One by one please.
• Use some kind of submodules to partition the codebase and tests <- 🆘 here, I don't know how.
• ...