Update required PostgreSQL in pgxn meta data

Support for pg12 has already been removed from the code, and 2.3.2
will only support pg13 and above.

Author: AndersAstrand

META.json

@@ -21,7 +21,7 @@
    "prereqs": {
       "runtime": {
          "requires": {
-            "PostgreSQL": "12.0.0"
+            "PostgreSQL": "13.0.0"
          }
       }
    },