Removing link to backports as it is broken (neo4j#762) (neo4j#767)

In 5.x, the link is not included anymore, thus the decision to remove it
here.

Cherry-picked from neo4j#762

Author: lidiazuin

modules/ROOT/pages/installation/linux/debian.adoc

@@ -12,12 +12,11 @@ To install Neo4j on Debian you need to make sure of the following:
 
 
 [[debian-ubuntu-prerequisites]]
-=== Java Prerequisites (Oracle Java, Debian 9+ and Ubuntu 16.04+ only)
+=== Java Prerequisites (Oracle Java and Ubuntu 16.04+ only)
 
 Neo4j {neo4j-version} requires the Java 11 runtime.
-Java 11 is not included in Ubuntu 16.04 LTS or Debian 9 (stretch) and will have to be set up manually prior to installing or upgrading to Neo4j {neo4j-version}, as described below.
-Debian 9 users can find OpenJDK 11 in https://packages.debian.org/stretch-backports/openjdk-11-jdk[backports].
-Debian 10 and Ubuntu 18.04 onwards already have the Openjdk Java 11 package available through `apt`.
+Java 11 is not included in Ubuntu 16.04 LTS and will have to be set up manually prior to installing or upgrading to Neo4j {neo4j-version}, as described below.
+Ubuntu 18.04 onwards already has the Openjdk Java 11 package available through `apt`.
 
 
 [[debian-oraclejava]]
@@ -28,13 +27,6 @@ The Debian installer may still be used, but it will install OpenJDK Java 11 in a
 
 This is due to changes in Oracle's Debian package manifest between Java versions 8 and 11.
 
-
-[[debian-java]]
-==== Java 11 on Debian 9
-
-Java 11 must be installed before installing Neo4j on Debian 9 (stretch) systems.
-If you do not already have Java 11 installed, run the following commands to install OpenJDK Java 11:
-
 [source, shell]
 ----
 echo "deb http://httpredir.debian.org/debian stretch-backports main" | sudo tee -a /etc/apt/sources.list.d/stretch-backports.list

modules/ROOT/pages/installation/requirements.adoc

@@ -82,7 +82,7 @@ For personal use and software development:
 | Operating System                        | Supported JDK
 | *MacOS 10.14+*                          | ZuluJDK 11
 | *Ubuntu Desktop 16.04+*                 | OpenJDK 11, OracleJDK 11, and ZuluJDK 11
-| *Debian 9+*                             | OpenJDK 11, OracleJDK 11, and ZuluJDK 11
+| *Debian 10+*                            | OpenJDK 11, OracleJDK 11, and ZuluJDK 11
 | *SuSE 15+*                              | Oracle JDK 11
 | *Windows 10*                            | OracleJDK 11 and ZuluJDK 11
 |===