chore: update configuration of Dokka versioning plugin

The 'dokkaHtml' task now removes the 'api/references/<version>/older' folder after generating the documentation.

build.gradle.kts

@@ -97,6 +97,7 @@ tasks.dokkaHtml {
                 from(dokkaDirectory)
                 into(rootDir.resolve("docs"))
             }
+            delete(apiReferencesDir.resolve(currentVersion).resolve("older"))
         }
     }
 }