Upgrade to Spring Security 5.1.8.RELEASE

Closes gh-20258

Author: snicoll

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -178,7 +178,7 @@
 		<spring-plugin.version>1.2.0.RELEASE</spring-plugin.version>
 		<spring-restdocs.version>2.0.4.RELEASE</spring-restdocs.version>
 		<spring-retry.version>1.2.5.RELEASE</spring-retry.version>
-		<spring-security.version>5.1.7.RELEASE</spring-security.version>
+		<spring-security.version>5.1.8.RELEASE</spring-security.version>
 		<spring-session-bom.version>Bean-SR8</spring-session-bom.version>
 		<spring-ws.version>3.0.8.RELEASE</spring-ws.version>
 		<sqlite-jdbc.version>3.25.2</sqlite-jdbc.version>