1.21.9 version, :)

fabric/src/main/java/com/lukasabbe/bookshelfinspector/ModMenu.java

@@ -1,3 +1,4 @@
+/*
 package com.lukasabbe.bookshelfinspector;
 
 import com.terraformersmc.modmenu.api.ConfigScreenFactory;
@@ -40,3 +41,4 @@ public class ModMenu implements ModMenuApi {
         };
     }
 }
+ */

fabric/src/main/resources/fabric.mod.json

@@ -20,9 +20,6 @@
         ],
         "client": [
             "com.lukasabbe.bookshelfinspector.BookshelfInspectorFabricClient"
-        ],
-        "modmenu": [
-            "com.lukasabbe.bookshelfinspector.ModMenu"
         ]
     },
     "mixins": [
@@ -31,7 +28,7 @@
     "depends": {
         "fabricloader": ">=${fabric_loader_version}",
         "fabric-api": "*",
-        "minecraft": "~${minecraft_version}",
+        "minecraft": "~1.21.9-beta.1",
         "java": ">=${java_version}"
     }
 }