Finalize [this-escape] warnings fixes in apache-httpclient (#20774)

Author: welandaz

modules/openapi-generator/src/main/resources/Java/libraries/apache-httpclient/ApiClient.mustache

@@ -466,7 +466,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
    * @param value The header's value
    * @return API client
    */
-  public ApiClient addDefaultHeader(String key, String value) {
+  public final ApiClient addDefaultHeader(String key, String value) {
     defaultHeaderMap.put(key, value);
     return this;
   }

samples/client/echo_api/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java

@@ -384,7 +384,7 @@ public ApiClient setTempFolderPath(String tempFolderPath) {
    * @param value The header's value
    * @return API client
    */
-  public ApiClient addDefaultHeader(String key, String value) {
+  public final ApiClient addDefaultHeader(String key, String value) {
     defaultHeaderMap.put(key, value);
     return this;
   }

samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java

@@ -477,7 +477,7 @@ public ApiClient setTempFolderPath(String tempFolderPath) {
    * @param value The header's value
    * @return API client
    */
-  public ApiClient addDefaultHeader(String key, String value) {
+  public final ApiClient addDefaultHeader(String key, String value) {
     defaultHeaderMap.put(key, value);
     return this;
   }