3.x: Try using https to access the reactive streams javadoc (#7535)

* Update build.gradle

* Self-hosting the Reactive Streams Javadoc

It was build with JDK 9 and lacks the `package-list` file that is required by JDK 8 Javadoc. 

I had to unpack the official `-javadoc.jar` from maven and copy-pasted the `elements-list` as `package-list`. Let's hope it works.

build.gradle

@@ -76,7 +76,7 @@ javadoc {
 
     options.links(
             "https://docs.oracle.com/javase/8/docs/api/",
-            "http://www.reactive-streams.org/reactive-streams-${reactiveStreamsVersion}-javadoc/"
+            "https://reactivex.io/RxJava/org.reactivestreams.javadoc/${reactiveStreamsVersion}/"
     )
 
     finalizedBy javadocCleanup