Polish some external documentation links

See gh-18384

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

@@ -1507,6 +1507,7 @@
 								<sources-root>${project.basedir}/src</sources-root>
 								<asciidoc-sources-root>${project.basedir}/src/main/asciidoc</asciidoc-sources-root>
 								<generated-resources-root>${project.basedir}/target/generated-resources</generated-resources-root>
+								<jetty-version>${jetty.version}</jetty-version>
 								<jooq-version>${jooq.version}</jooq-version>
 								<spring-boot-version>${revision}</spring-boot-version>
 								<spring-amqp-version>${spring-amqp.version}</spring-amqp-version>

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

@@ -94,10 +94,10 @@
 
 :ant-docs: https://ant.apache.org/manual
 :dependency-management-plugin-code: https://github.com/spring-gradle-plugins/dependency-management-plugin
-:gradle-docs: https://docs.gradle.org/4.2.1/userguide
+:gradle-docs: https://docs.gradle.org/current/userguide
-:hibernate-docs: https://docs.jboss.org/hibernate/orm/5.3/userguide/html_single/Hibernate_User_Guide.html
+:hibernate-docs: https://docs.jboss.org/hibernate/orm/5.4/userguide/html_single/Hibernate_User_Guide.html
 :java-api: https://docs.oracle.com/javase/8/docs/api/
-:jetty-docs: https://www.eclipse.org/jetty/documentation/9.4.x
+:jetty-docs: https://www.eclipse.org/jetty/documentation/{jetty-version}
 :jooq-docs: https://www.jooq.org/doc/{jooq-version}/manual-single-page
 :junit5-docs: https://junit.org/junit5/docs/current/user-guide
 :kotlin-docs: https://kotlinlang.org/docs/reference/