Update flow to 24.7.0.alpha15 (#7352)

Author: vaadin-bot

versions.json

@@ -122,7 +122,7 @@
             "npmName": "@vaadin/field-highlighter"
         },
         "flow": {
-            "javaVersion": "24.7-SNAPSHOT"
+            "javaVersion": "24.7.0.alpha15"
         },
         "flow-cdi": {
             "javaVersion": "15.1.0"