Include 2.6.x branch in Antora docs

Issue gh-2014

Author: eleftherias

spring-session-docs/antora-playbook.yml

@@ -9,7 +9,7 @@ content:
     - url: https://github.com/spring-io/spring-generated-docs
       branches: [spring-projects/spring-session/*]
     - url: https://github.com/spring-projects/spring-session
-      branches: [main,3.0.x]
+      branches: [main,3.0.x,2.6.x]
       tags: [ '3.0.*', '2.7.*', '2.6.*','!2.6.0-M*','!2.6.0-RC*','!2.7.0-M1','!3.0.0-M1']
       start_path: spring-session-docs
 urls: