Fix KAFKA-7789 by increasing the key size for the RSA keys generated for (#6096)

Reviewers: Jun Rao <junrao@gmail.com>

clients/src/test/java/org/apache/kafka/test/TestSslUtils.java

@@ -89,7 +89,7 @@ public class TestSslUtils {
 
     public static KeyPair generateKeyPair(String algorithm) throws NoSuchAlgorithmException {
         KeyPairGenerator keyGen = KeyPairGenerator.getInstance(algorithm);
-        keyGen.initialize(1024);
+        keyGen.initialize(2048);
         return keyGen.genKeyPair();
     }