Update links to Spring Security's reference docs

Closes gh-28618

spring-boot-project/spring-boot-docs/build.gradle

@@ -257,7 +257,9 @@ tasks.withType(org.asciidoctor.gradle.jvm.AbstractAsciidoctorTask) {
 	doFirst {
 		def versionConstraints = dependencyVersions.versionConstraints
 		def securityVersion = versionConstraints["org.springframework.security:spring-security-core"]
-		securityVersion = securityVersion.substring(0, securityVersion.lastIndexOf("."))
+		if (securityVersion.endsWith("-SNAPSHOT")) {
+			securityVersion = securityVersion.substring(0, securityVersion.length() - "-SNAPSHOT".length())
+		}
 		attributes	"embedded-mongo-version": versionConstraints["de.flapdoodle.embed:de.flapdoodle.embed.mongo"],
 					"jetty-version": versionConstraints["org.eclipse.jetty:jetty-server"],
 					"jooq-version": versionConstraints["org.jooq:jooq"],

spring-boot-project/spring-boot-docs/src/docs/asciidoc/attributes.adoc

@@ -86,7 +86,7 @@
 :spring-kafka-docs: https://docs.spring.io/spring-kafka/docs/{spring-kafka-version}/reference/html/
 :spring-restdocs: https://spring.io/projects/spring-restdocs
 :spring-security: https://spring.io/projects/spring-security
-:spring-security-docs: https://docs.spring.io/spring-security/{spring-security-version}
+:spring-security-docs: https://docs.spring.io/spring-security/reference/{spring-security-version}
 :spring-security-oauth2: https://spring.io/projects/spring-security-oauth
 :spring-security-oauth2-docs: https://projects.spring.io/spring-security-oauth/docs/oauth2.html
 :spring-session: https://spring.io/projects/spring-session