Replace remaining references to Elasticsearch Service / ESS with current naming

[florent-leborgne]

Replaces:

• {{ess}} with the appropriate {{ech}} or {{ecloud}} depending on the context
• Same for plain references of Elasticsearch Service
• The few remaining occurrences of Elasticsearch service are either made on purpose ("formerly known as..."), or are referring to the self-managed Elasticsearch "service" (not capitalized) in install docs.

[georgewallace]

LGTM

[leemthompo]

Spot check passes the smell test 👃 🧹

[kilfoyle]

LGTM! 🦖
(Couldn't resist dropping in to have a look at these nice changes.)