[DOCS] Update ES highlights include for 7.4 release (#569)

Author: jrodewig

docs/en/install-upgrade/highlights.asciidoc

@@ -46,7 +46,7 @@ For the complete list, go to {ref}/release-highlights.html[{es} release highligh
 ////
 :leveloffset: +1
 
-include::{es-repo-dir}/reference/release-notes/highlights-7.3.0.asciidoc[tag=notable-highlights]
+include::{es-repo-dir}/reference/release-notes/highlights-7.4.0.asciidoc[tag=notable-highlights]
 
 :leveloffset: -1
 ////