fix: remove pg18 from pgxn test

Author: frectonz

.github/workflows/build.yml

@@ -9,7 +9,7 @@ jobs:
     container: pgxn/pgxn-tools
     strategy:
       matrix:
-        pg: [13, 14, 15, 16, 17, 18]
+        pg: [13, 14, 15, 16, 17]
     name: 🐘 Postgres ${{ matrix.pg }}
     steps:
       - name: Checkout