add details about polymer

Author: tepi

articles/upgrading/index.adoc

@@ -294,7 +294,7 @@ java {
 
 == Application Servers
 
-Before migrating, find the corresponding version of the Jakarta EE 11-compatible application server used in your project. See https://jakarta.ee/compatibility/[Jakarta Compatible Products] for more information.
+Before migrating, find the corresponding version of the Jakarta EE 11-compatible application server used in your project. See link:https://jakarta.ee/compatibility/[Jakarta Compatible Products] for more information.
 
 
 
@@ -305,7 +305,7 @@ Vaadin uses the [filename]`{project directory}/src/main/frontend/` directory as
 
 == Polymer Support
 
-Polymer dependency no longer included by default. <TODO: add details>
+In Vaadin 25, the `@polymer/polymer` dependency in default `package.json` is removed by default, if polymer-template module is not found from the project. If the application uses Polymer in add-ons may require to add `@NpmPackage(value = "@polymer/polymer", version = "3.5.2")` or add an import to `package.json` explicitly. Details can be found in link:https://github.com/vaadin/flow/issues/21421[Ensure Flow works without Polymer in v25]
 
 
 == Maven & Gradle Plugins