Add extra note about Java8 requirement to sub-install docs (#21190)

Closes #20005

docs/reference/setup/install/deb.asciidoc

@@ -9,6 +9,10 @@ The latest stable version of Elasticsearch can be found on the
 link:/downloads/elasticsearch[Download Elasticsearch] page. Other versions can
 be found on the link:/downloads/past-releases[Past Releases page].
 
+NOTE: Elasticsearch requires Java 8 or later. Use the 
+http://www.oracle.com/technetwork/java/javase/downloads/index.html[official Oracle distribution]
+or an open-source distribution such as http://openjdk.java.net[OpenJDK].
+
 [[deb-key]]
 ==== Import the Elasticsearch PGP Key
 

docs/reference/setup/install/rpm.asciidoc

@@ -13,6 +13,10 @@ The latest stable version of Elasticsearch can be found on the
 link:/downloads/elasticsearch[Download Elasticsearch] page. Other versions can
 be found on the link:/downloads/past-releases[Past Releases page].
 
+NOTE: Elasticsearch requires Java 8 or later. Use the
+http://www.oracle.com/technetwork/java/javase/downloads/index.html[official Oracle distribution]
+or an open-source distribution such as http://openjdk.java.net[OpenJDK].
+
 [[rpm-key]]
 ==== Import the Elasticsearch PGP Key
 

docs/reference/setup/install/windows.asciidoc

@@ -10,6 +10,10 @@ link:/downloads/elasticsearch[Download Elasticsearch] page.
 Other versions can be found on the
 link:/downloads/past-releases[Past Releases page].
 
+NOTE: Elasticsearch requires Java 8 or later. Use the
+http://www.oracle.com/technetwork/java/javase/downloads/index.html[official Oracle distribution]
+or an open-source distribution such as http://openjdk.java.net[OpenJDK].
+
 [[install-windows]]
 ==== Download and install the `.zip` package
 

docs/reference/setup/install/zip-targz.asciidoc

@@ -10,6 +10,10 @@ link:/downloads/elasticsearch[Download Elasticsearch] page.
 Other versions can be found on the
 link:/downloads/past-releases[Past Releases page].
 
+NOTE: Elasticsearch requires Java 8 or later. Use the
+http://www.oracle.com/technetwork/java/javase/downloads/index.html[official Oracle distribution]
+or an open-source distribution such as http://openjdk.java.net[OpenJDK].
+
 [[install-zip]]
 ==== Download and install the `.zip` package
 
@@ -185,4 +189,4 @@ directory so that you do not delete important data later on.
 |=======================================================================
 
 
-include::next-steps.asciidoc[]
+include::next-steps.asciidoc[]