[FLINK-36170] bump assert4j version

Author: r-sidd

pom.xml

@@ -96,7 +96,7 @@ under the License.
         <okhttp.version>4.12.0</okhttp.version>
         <curator-test.version>5.2.0</curator-test.version>
 
-        <assertj.version>3.21.0</assertj.version>
+        <assertj.version>3.26.3</assertj.version>
 
         <quartz.version>2.3.2</quartz.version>
         <hikari.version>5.1.0</hikari.version>