Make CI fetch stdlib 2.3

Author: Taneb

.github/workflows/ci-ubuntu.yml

@@ -32,7 +32,7 @@ on:
 
 env:
   AGDA_VERSION: "2.7.0.1"
-  STDLIB_VERSION: "2.1"
+  STDLIB_VERSION: "2.3"
 
   AGDA: agda --auto-inline -Werror +RTS -M6G -H3.5G -A128M -RTS -i . -i src/