Upgrade to Gradle 9 (#9896)

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Co-authored-by: Dmitrii Shmatko <zilm@zilm.net>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Enrico Del Fante <enrico.delfante@consensys.net>
Co-authored-by: Dmitrii Shmatko <zilm13@users.noreply.github.com>

Author: 4 people

beacon/validator/build.gradle

@@ -1,6 +1,6 @@
 idea {
   module {
-    testSourceDirs += sourceSets.jmh.java.srcDirs
+    testSources.from(sourceSets.jmh.java.srcDirs)
   }
 }