fix(pg_graphql): use string comparison for pgVersion

pgVersion is a string, so comparison should be string-to-string
instead of string-to-integer.

Author: yvan-sraka

nix/ext/pg_graphql/default.nix

@@ -164,7 +164,7 @@ let
     _: value: !builtins.elem pgVersion value.postgresql
   ) allVersions;
   unsupportedPackages =
-    if pgVersion == 15 then
+    if pgVersion == "15" then
       [ ]
     else
       # Include SQL-only packages for PG15 extension versions incompatible with current PG