Operators that work with Context should not allow replacing it

[bsideup]

Motivation

Currently, there are operators like retryWhen that allow replacing the Context in the user's code. It is error prone and does not seem to have valid use cases (to be validated)

Desired solution

Instead of replacing, user-provided Context should always be merged into the current one.
Currently, only retryWhen is considered, but we should check whether there are other operators that allow user-provided functions replace the Context.

Considered alternatives

TBD

Additional context

https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html#retryWhen-java.util.function.Function-

Note that if the companion Publisher created by the whenFactory emits Context as trigger objects, these Context will REPLACE the operator's own Context. Please be careful there: replacing the Context means that some keys you don't own could be removed, breaking libraries that depend on them.