Drop back to Surefire 2.19.2 in case 2.20 has a memory leak

See gh-6299

spring-boot-dependencies/pom.xml

@@ -213,7 +213,7 @@
 		<maven-shade-plugin.version>2.4.3</maven-shade-plugin.version>
 		<maven-site-plugin.version>3.5.1</maven-site-plugin.version>
 		<maven-source-plugin.version>3.0.1</maven-source-plugin.version>
-		<maven-surefire-plugin.version>2.20</maven-surefire-plugin.version>
+		<maven-surefire-plugin.version>2.19.2</maven-surefire-plugin.version>
 		<maven-war-plugin.version>3.1.0</maven-war-plugin.version>
 		<versions-maven-plugin.version>2.3</versions-maven-plugin.version>
 	</properties>