This commit introduces a dedicated envirnoment variable ES_JAVA_HOME to
determine the JDK used to start (if not using the bundled JDK). This
environment variable will replace JAVA_HOME. The reason that we are
making this change is because JAVA_HOME is a common environment variable
and sometimes users have it set in their environment from other JDK
applications that they have installed on their system. In this case,
they would accidentally end up not using the bundled JDK despite their
intentions. By using a dedicated environment variable specific to
Elasticsearch, we avoid this potential for conflict. With this commit,
we introduce the new environment variable, and deprecate the use of
JAVA_HOME. We will remove support for JAVA_HOME in a future commit.
0cd4863585