Use 6.0 for generated docs

docs/spring-security-docs.gradle

@@ -26,7 +26,7 @@ tasks.register("generateAntora") {
 		outputFile.createNewFile()
 		outputFile.setText("""name: ROOT
 title: Spring Security
-version: 5.6
+version: 6.0
 start_page: ROOT:index.adoc
 asciidoc:
   attributes: