Update to Antora 3.1.0

Author: rwinch

spring-session-docs/spring-session-docs.gradle

@@ -29,7 +29,7 @@ dependencies {
 }
 
 antora {
-    antoraVersion = "3.0.1"
+    antoraVersion = "3.1.0"
     arguments = ["--fetch"]
 }