Correct additions to spring-boot-starter-parent's pom

Following the fix for gh-21989, spring-boot-starter-parent no longer
contains an <issueManagement> element. As a result the additional
content was no longer being added to the pom. This commit updates
the additions so that they are now added after the <scm> element
that is still present.

See gh-21989

spring-boot-project/spring-boot-starters/spring-boot-starter-parent/build.gradle

@@ -27,7 +27,7 @@ publishing.publications.withType(MavenPublication) {
 				delegate."project.reporting.outputEncoding"('UTF-8')
 			}
 		}
-		root.issueManagement.plus {
+		root.scm.plus {
 			build {
 				resources {
 					resource {