Upgrade Oracle Docker image to version 21.3.0-slim

Closes #1268

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/testing/OracleDataSourceConfiguration.java

@@ -50,7 +50,7 @@ public class OracleDataSourceConfiguration extends DataSourceConfiguration {
 		if (ORACLE_CONTAINER == null) {
 
 			LOG.info("Oracle starting...");
-			OracleContainer container = new OracleContainer("gvenzl/oracle-xe:18.4.0-slim").withReuse(true);
+			OracleContainer container = new OracleContainer("gvenzl/oracle-xe:21.3.0-slim").withReuse(true);
 			container.start();
 			LOG.info("Oracle started");
 

spring-data-r2dbc/src/test/java/org/springframework/data/r2dbc/testing/OracleTestSupport.java

@@ -125,7 +125,7 @@ public class OracleTestSupport {
 		if (testContainerDatabase == null) {
 
 			try {
-				OracleContainer container = new OracleContainer("gvenzl/oracle-xe:18.4.0-slim")
+				OracleContainer container = new OracleContainer("gvenzl/oracle-xe:21.3.0-slim")
 						.withReuse(true);
 				container.start();