Update changelog

Author: hpehl

CHANGELOG.md

@@ -7,6 +7,11 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Changed
+
+- Elemento now requires Java 21.\
+  GWT applications can still be used with Elemento: If you use the [GWT Maven plugin](https://tbroyer.github.io/gwt-maven-plugin/index.html), set the `sourceLevel` to the according language level. See the [flow sample](https://github.com/hal/elemento/blob/main/samples/flow/pom.xml#L98) for an example.
+
 ### Upgrades
 
 - Bump Elemental2 to 1.3.2

samples/pom.xml

@@ -34,8 +34,10 @@
     <properties>
         <!-- Dependency versions -->
         <version.gwt>2.12.2</version.gwt>
+        <version.kie.j2cl.bom>0.4</version.kie.j2cl.bom>
         <version.node>v24.10.0</version.node>
         <version.j2cl.platform.bom>20251001-1</version.j2cl.platform.bom>
+        <version.treblereel.j2cl.processors>0.8</version.treblereel.j2cl.processors>
 
         <!-- Plugin versions -->
         <version.frontend.plugin>1.15.4</version.frontend.plugin>
@@ -88,6 +90,13 @@
                 <version>${project.version}</version>
                 <classifier>resources</classifier>
             </dependency>
+            <dependency>
+                <groupId>org.kie.j2cl.tools</groupId>
+                <artifactId>bom</artifactId>
+                <version>${version.kie.j2cl.bom}</version>
+                <type>pom</type>
+                <scope>import</scope>
+            </dependency>
             <dependency>
                 <groupId>org.treblereel.j2cl.platform</groupId>
                 <artifactId>bom</artifactId>