Update version number (elastic#2217)

Author: benironside

docs/en/install-upgrade/breaking.asciidoc

@@ -67,9 +67,9 @@ the complete list, go to {ref}/breaking-changes.html[{es} breaking changes].
 ++++
 
 This list summarizes the most important breaking changes in {elastic-sec} {version}. For
-the complete list, go to {security-guide-all}/8.3/release-notes-header-8.3.0.html#breaking-changes-8.3.0[{elastic-sec} breaking changes].
+the complete list, go to {security-guide-all}/8.4/release-notes-header-8.4.0.html#breaking-changes-8.4.0[{elastic-sec} breaking changes].
 
-include::{security-repo-dir}/release-notes/8.3.asciidoc[tag=breaking-changes]
+include::{security-repo-dir}/release-notes/8.4.asciidoc[tag=breaking-changes]
 
 [[kibana-breaking-changes]]
 === {kib} breaking changes