Upgrade to latest version of Asciidoctor

Upgrade to latest version of Asciidoctor because we were several
versions behind and were steadily incurring more technical debt.

* Replace project-version attribute in Asciidoctor

I had removed the project-version attribute, but the docs use it all
over, so I put it back.

* Removed unneeded repositories

It turns out that those repositories are not needed.

A cross-reference was incorrectly formatted (missing the # character).

src/reference/asciidoc/index-header.adoc

@@ -3,7 +3,7 @@
 Mark Fisher; Marius Bogoevici; Iwein Fuld; Jonas Partner; Oleg Zhurakousky; Gary Russell; Dave Syer; Josh Long; David Turanski; Gunnar Hillert; Artem Bilan; Amol Nayak; Jay Bryant
 
 ifdef::backend-html5[]
-*{project-version}*
+*{revnumber}*
 endif::[]
 
 (C) 2009 - 2020 Pivotal Software, Inc.

src/reference/asciidoc/mongodb.adoc

@@ -548,7 +548,7 @@ Starting with version 5.3, the `ReactiveMongoDbStoringMessageHandler` and `React
 They are based on the `ReactiveMongoOperations` from Spring Data and requires a `org.mongodb:mongodb-driver-reactivestreams` dependency.
 
 The `ReactiveMongoDbStoringMessageHandler` is an implementation of the `ReactiveMessageHandler` which is supported natively in the framework when reactive streams composition is involved in the integration flow definition.
-See more information in the <<./reactive-streams.adoc/reactive-message-handler,ReactiveMessageHandler>>.
+See more information in the <<./reactive-streams.adoc#reactive-message-handler,ReactiveMessageHandler>>.
 
 From configuration perspective there is no difference with many other standard channel adapters.
 For example with Java DSL such a channel adapter could be used like: