Upgrade to testcontainers 1.21.4 (modelcontextprotocol#743)

To fix a "docker-machine executable was not found on PATH"
error with recent Docker versions (impact at least Docker
4.55 and 4.56).

Author: sdeleuze

pom.xml

@@ -62,7 +62,7 @@
 		<assert4j.version>3.27.6</assert4j.version>
 		<junit.version>5.10.2</junit.version>
 		<mockito.version>5.20.0</mockito.version>
-		<testcontainers.version>1.20.4</testcontainers.version>
+		<testcontainers.version>1.21.4</testcontainers.version>
 		<byte-buddy.version>1.17.8</byte-buddy.version>
 		<toxiproxy.version>1.21.0</toxiproxy.version>