updates reactor-core dependency for the feign-reactive-wrappers

[nlsbchmnn]

removed deprecated usage of Schedulers.elastic() and using Schedulers.boundedElastic() instead

Fixes #1711

[velo]

Problem is this breaks binary compatibility.... is so annoying when projects break compatibility on minor releases

[velo]

can we make Scheduler scheduler a parameter, and if never set, we default to Schedulers.boundedElastic()?

And then document how to use 3.4

[nlsbchmnn]

Just for my understanding: Why is it breaking binary compatibility?

It appears to me that the scheduler is already a parameter, which you can set with public Builder scheduleOn(Scheduler scheduler) and Schedulers.elastic() is the current default.

[velo]

elastic to boundedElastic breaks anyone using the old version of the library.

Tell you what, as a compromise, move the initialization to constructor and have one that allows user to pick how to create the scheduler and the other defaults to new code

  public static Builder builder() {
    return new Builder(Schedulers.boundedElastic());
  }

  public static Builder builder(Scheduler scheduler) {
    return new Builder(scheduler);
  }

  public static class Builder extends ReactiveFeign.Builder {

    private Scheduler scheduler;

.. constructor...