Downgrade to maven-failsafe-plugin 2.18.1

This commit downgrades the maven failsafe plugin version. As of 2.19, the
generated jar is used for integration tests rather than the
`target/classes` directory. The fat jar in 1.4 has changed to move
classes to `BOOT-INF/classes` and therefore these are not found anymore.

There isn't a way to configure the jar file to use in the plugin. Users
willing to use the latest version should configure a `classifier` for the
repackaged jar so that the original jar is used by failsafe.

Closes gh-6254

spring-boot-dependencies/pom.xml

@@ -2434,7 +2434,7 @@
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-failsafe-plugin</artifactId>
-					<version>2.19.1</version>
+					<version>2.18.1</version>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>