This commit overhauls the documentation of discovery and cluster coordination,
removing mention of the Zen Discovery module and replacing it with docs for the
new cluster coordination mechanism introduced in 7.0.
Relates #32006
* Clean up s/ElasticSearch/Elasticsearch on docs/*
* Clean up s/ElasticSearch/Elasticsearch on src/* bin/* & pom.xml
* Clean up s/ElasticSearch/Elasticsearch on NOTICE.txt and README.textile
Closes#4634