Update some references to 2.4.x where appropriate

See gh-21612

README.adoc

@@ -1,4 +1,4 @@
-= Spring Boot image:https://ci.spring.io/api/v1/teams/spring-boot/pipelines/spring-boot-2.3.x/jobs/build/badge["Build Status", link="https://ci.spring.io/teams/spring-boot/pipelines/spring-boot-2.3.x?groups=Build"] image:https://badges.gitter.im/Join Chat.svg["Chat",link="https://gitter.im/spring-projects/spring-boot?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"]
+= Spring Boot image:https://ci.spring.io/api/v1/teams/spring-boot/pipelines/spring-boot-2.4.x/jobs/build/badge["Build Status", link="https://ci.spring.io/teams/spring-boot/pipelines/spring-boot-2.4.x?groups=Build"] image:https://badges.gitter.im/Join Chat.svg["Chat",link="https://gitter.im/spring-projects/spring-boot?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"]
 :docs: https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference
 :github: https://github.com/spring-projects/spring-boot
 

eclipse/spring-boot-project.setup

@@ -11,8 +11,8 @@
     xmlns:setup.workingsets="http://www.eclipse.org/oomph/setup/workingsets/1.0"
     xmlns:workingsets="http://www.eclipse.org/oomph/workingsets/1.0"
     xsi:schemaLocation="http://www.eclipse.org/oomph/setup/jdt/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/JDT.ecore http://www.eclipse.org/buildship/oomph/1.0 https://raw.githubusercontent.com/eclipse/buildship/master/org.eclipse.buildship.oomph/model/GradleImport-1.0.ecore http://www.eclipse.org/oomph/predicates/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/Predicates.ecore http://www.eclipse.org/oomph/setup/workingsets/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/SetupWorkingSets.ecore http://www.eclipse.org/oomph/workingsets/1.0 http://git.eclipse.org/c/oomph/org.eclipse.oomph.git/plain/setups/models/WorkingSets.ecore"
-    name="spring.boot.2.3.x"
-    label="Spring Boot 2.3.x">
+    name="spring.boot.2.4.x"
+    label="Spring Boot 2.4.x">
   <setupTask
       xsi:type="setup:VariableTask"
       type="FOLDER"