[FLINK-39098] Bump testcontainers to 1.21.4 for Docker API compatibility

Author: ferenc-csaky

pom.xml

@@ -59,7 +59,7 @@ under the License.
 		<assertj.version>3.27.7</assertj.version>
 		<junit5.version>5.10.2</junit5.version>
 		<mockito.version>1.10.19</mockito.version>
-		<testcontainers.version>1.17.6</testcontainers.version>
+		<testcontainers.version>1.21.4</testcontainers.version>
 
 		<log4j.version>2.25.3</log4j.version>
 		<slf4j.version>1.7.36</slf4j.version>