version bumped to 8.15.4

Author: eedugon

shared/versions/stack/8.15.asciidoc

@@ -1,12 +1,12 @@
-:version:                8.15.3
+:version:                8.15.4
 ////
 bare_version never includes -alpha or -beta
 ////
-:bare_version:           8.15.3
-:logstash_version:       8.15.3
-:elasticsearch_version:  8.15.3
-:kibana_version:         8.15.3
-:apm_server_version:     8.15.3
+:bare_version:           8.15.4
+:logstash_version:       8.15.4
+:elasticsearch_version:  8.15.4
+:kibana_version:         8.15.4
+:apm_server_version:     8.15.4
 :branch:                 8.15
 :minor-version:          8.15
 :major-version:          8.x