Downgraded agda version

The specified version was not available on CI

.github/workflows/ci.yaml

@@ -13,13 +13,13 @@ jobs:
     - name: 📥 Checkout repository
       uses: actions/checkout@v4
 
-    # Setup Agda 2.6.4.1 with version 2.0 of agda-stdlib and
+    # Setup Agda 2.6.3 with recommended version of agda-stdlib and
     # uses standard-library as default
     - name: ❄ Setup Agda
       uses: wenkokke/setup-agda@v2
       with:
-        agda-version: '2.6.4.1'
-        agda-stdlib-version: '2.0'
+        agda-version: '2.6.3'
+        agda-stdlib-version: 'recommended'
         agda-defaults: |
           standard-library
         agda-libraries: |