Update confluent version to fix CVE-2024-26308 CVE-2024-25710 (#32674)

* bump confluent version

Kafka Schema Registry Client has been reported with following vuln 
CVE-2024-26308
CVE-2024-25710 due to vulnerable dependencies.

* try slighly older version due to unmet dependencies to ThrottlingQuotaExceededException

* try slighly older version due to unmet dependencies to ThrottlingQuotaExceededException

* comment on version

sdks/java/io/kafka/build.gradle

@@ -31,7 +31,8 @@ enableJavaPerformanceTesting()
 description = "Apache Beam :: SDKs :: Java :: IO :: Kafka"
 ext {
     summary = "Library to read Kafka topics."
-    confluentVersion = "7.6.0"
+    // newer versions e.g. 7.6.* require dropping support for older kafka versions.
+    confluentVersion = "7.5.5"
 }
 
 def kafkaVersions = [