[DOCS] Edits 7.3 release highlights

[lcawl]

This PR updates the Elasticsearch 7.3.0 Release Highlights so that they can be successfully re-used in the Installation and Upgrade Guide (https://www.elastic.co/guide/en/elastic-stack/current/elasticsearch-highlights.html). In particular, it:

• removes the coming[7.3.0] tag,
• moves the es-sql attribute to the shared attributes in https://github.com/elastic/docs/blob/master/shared/attributes.asciidoc
• change all inline links to external links, so they'll work in both places

[elasticmachine]

Pinging @elastic/es-docs

[debadair]

LGTM