Applying PR review suggestions

Author: LikeTheSalad

opamp-client/src/main/java/io/opentelemetry/opamp/client/internal/connectivity/http/HttpErrorException.java

@@ -7,9 +7,10 @@
 
 import java.util.Objects;
 
-@SuppressWarnings("serial")
 public class HttpErrorException extends Exception {
-  public final int errorCode;
+  private final int errorCode;
+
+  private static final long serialVersionUID = 1L;
 
   @Override
   public boolean equals(Object o) {