Fix any prerelease that != true or undefined

Previously the build would fail with duplicate versions because milestones
and RCs had the same version with a different prerelease value.

Now this is patched by changing the version to contain any prerelease value
other than true and changing prerelease to true.

lib/antora/extensions/version-fix.js

@@ -12,12 +12,16 @@ module.exports.register = function({ config }) {
                 aggregate.displayVersion = `${aggregate.version}`
                 delete aggregate.prerelease
             }
-            if (aggregate.version === "5.6.1" &&
+            else if (aggregate.version === "5.6.1" &&
                     aggregate.prerelease == "-SNAPSHOT") {
                 aggregate.version = "5.6.1"
                 aggregate.displayVersion = `${aggregate.version}`
                 delete aggregate.prerelease
             }
+            else if (aggregate.prerelease && aggregate.prerelease != true) {
+                aggregate.version += aggregate.prerelease
+                aggregate.prerelease = true
+            }
         })
     })
 }