Update to Gradle 5.0

Change project's gradle version to 5.0, this requires to make some minor
adjustments.

Fixes: gh-6148

config/spring-security-config.gradle

@@ -73,9 +73,6 @@ dependencies {
 	testRuntime 'org.hsqldb:hsqldb'
 }
 
-test {
-	inputs.file file("$rootDir/docs/manual/src/docbook/appendix-namespace.xml")
-}
 
 rncToXsd {
 	rncDir = file('src/main/resources/org/springframework/security/config/')