chore: Rename ELK_VERSION to ELASTIC_VERSION

More appropriate name considering that extensions, which aren't part of
the "ELK" stack, also rely on this variable.

elasticsearch/Dockerfile

@@ -1,7 +1,7 @@
-ARG ELK_VERSION
+ARG ELASTIC_VERSION
 
 # https://www.docker.elastic.co/
-FROM docker.elastic.co/elasticsearch/elasticsearch:${ELK_VERSION}
+FROM docker.elastic.co/elasticsearch/elasticsearch:${ELASTIC_VERSION}
 
 # Add your elasticsearch plugins setup here
 # Example: RUN elasticsearch-plugin install analysis-icu