feat: Drop support for pg 9.6

nix/overlays/postgresql-legacy.nix

@@ -2,21 +2,6 @@ _: _:
 # Overlay that adds legacy versions of PostgreSQL that are supported by
 # PostgREST.
 {
-  # PostgreSQL 9.6 was removed from Nixpkgs with
-  # https://github.com/NixOS/nixpkgs/commit/757dd008b2f2926fc0f7688fa8189f930ea47521
-  # We pin its parent commit to get the last version that was available.
-  postgresql_9_6 =
-    let
-      rev = "571cbf3d1db477058303cef8754fb85a14e90eb7";
-      tarballHash = "0q74wn418i1bn5sssacmw8ykpmqvzr0s93sj6pbs3rf6bf134fkz";
-      pinnedPkgs =
-        builtins.fetchTarball {
-          url = "https://github.com/nixos/nixpkgs/archive/${rev}.tar.gz";
-          sha256 = tarballHash;
-        };
-    in
-    (import pinnedPkgs { }).pkgs.postgresql_9_6;
-
   # PostgreSQL 10 was removed from Nixpkgs with
   # https://github.com/NixOS/nixpkgs/commit/aa1483114bb329fee7e1266100b8d8921ed4723f
   # We pin its parent commit to get the last version that was available.