chore: reduce logging when auth is successful

this is constantly logged without any action needed. Can switch to DEBUG to reduce logging impact.

src/main/java/io/aiven/kafka/auth/AivenSaslPlainServerCallbackHandler.java

@@ -93,7 +93,7 @@ public boolean authenticate(final String username, final char[] password) {
                             LOGGER.error("Authentication failed for {}, no password set", username);
                             return false;
                         } else if (storedPassword.equals(strPassword)) {
-                            LOGGER.info("Authentication successful for {}", username);
+                            LOGGER.debug("Authentication successful for {}", username);
                             return true;
                         } else {
                             LOGGER.error("Authentication failed for {}, invalid password", username);