Instead of deleting aggregate.prerelease set to true

This ensures that the aggregate is not marked as a stable version.

lib/antora/extensions/version-fix.js

@@ -10,13 +10,13 @@ module.exports.register = function({ config }) {
                 aggregate.version = "5.6.0-RC1"
                 aggregate.startPage = "ROOT:index.adoc"
                 aggregate.displayVersion = `${aggregate.version}`
-                delete aggregate.prerelease
+                aggregate.prerelease = true
             }
             else if (aggregate.version === "5.6.1" &&
                     aggregate.prerelease == "-SNAPSHOT") {
                 aggregate.version = "5.6.1"
                 aggregate.displayVersion = `${aggregate.version}`
-                delete aggregate.prerelease
+                aggregate.prerelease = true
             }
             else if (aggregate.prerelease && aggregate.prerelease != true) {
                 aggregate.version += aggregate.prerelease