Update Hilla to pre-release 24.8.0.alpha6

because of Kotlin nullability support

build.gradle.kts

@@ -6,6 +6,9 @@ allprojects {
     repositories {
         mavenCentral()
         mavenLocal()
+        maven {
+            setUrl("https://maven.vaadin.com/vaadin-prereleases/")
+        }
     }
 }