Use older package-set for purescript 0.15.6

Going with purs 0.15.6 because of an open CI issue with 0.15.7.
Therefore we need to use an older package-set.

packages.dhall

@@ -1,106 +1,6 @@
-{-
-Welcome to your new Dhall package-set!
-
-Below are instructions for how to edit this file for most use
-cases, so that you don't need to know Dhall to use it.
-
-## Use Cases
-
-Most will want to do one or both of these options:
-1. Override/Patch a package's dependency
-2. Add a package not already in the default package set
-
-This file will continue to work whether you use one or both options.
-Instructions for each option are explained below.
-
-### Overriding/Patching a package
-
-Purpose:
-- Change a package's dependency to a newer/older release than the
-    default package set's release
-- Use your own modified version of some dependency that may
-    include new API, changed API, removed API by
-    using your custom git repo of the library rather than
-    the package set's repo
-
-Syntax:
-where `entityName` is one of the following:
-- dependencies
-- repo
-- version
--------------------------------
-let upstream = --
-in  upstream
-  with packageName.entityName = "new value"
--------------------------------
-
-Example:
--------------------------------
-let upstream = --
-in  upstream
-  with halogen.version = "master"
-  with halogen.repo = "https://example.com/path/to/git/repo.git"
-
-  with halogen-vdom.version = "v4.0.0"
-  with halogen-vdom.dependencies = [ "extra-dependency" ] # halogen-vdom.dependencies
--------------------------------
-
-### Additions
-
-Purpose:
-- Add packages that aren't already included in the default package set
-
-Syntax:
-where `<version>` is:
-- a tag (i.e. "v4.0.0")
-- a branch (i.e. "master")
-- commit hash (i.e. "701f3e44aafb1a6459281714858fadf2c4c2a977")
--------------------------------
-let upstream = --
-in  upstream
-  with new-package-name =
-    { dependencies =
-       [ "dependency1"
-       , "dependency2"
-       ]
-    , repo =
-       "https://example.com/path/to/git/repo.git"
-    , version =
-        "<version>"
-    }
--------------------------------
-
-Example:
--------------------------------
-let upstream = --
-in  upstream
-  with benchotron =
-      { dependencies =
-          [ "arrays"
-          , "exists"
-          , "profunctor"
-          , "strings"
-          , "quickcheck"
-          , "lcg"
-          , "transformers"
-          , "foldable-traversable"
-          , "exceptions"
-          , "node-fs"
-          , "node-buffer"
-          , "node-readline"
-          , "datetime"
-          , "now"
-          ]
-      , repo =
-          "https://github.com/hdgarrood/purescript-benchotron.git"
-      , version =
-          "v7.0.0"
-      }
--------------------------------
--}
 let upstream =
-      https://github.com/purescript/package-sets/releases/download/psc-0.15.7-20230112/packages.dhall
-        sha256:3e5a439b9975949016eac07660ea2c80531ef0eb08903fcbeacc1d291a05cea0
+      https://github.com/purescript/package-sets/releases/download/psc-0.15.4-20230105/packages.dhall
+        sha256:3e9fbc9ba03e9a1fcfd895f65e2d50ee2f5e86c4cd273f3d5c841b655a0e1bda
 
 in  upstream
   with nextjs =

spago.dhall

@@ -1,15 +1,3 @@
-{-
-Welcome to a Spago project!
-You can edit this file as you like.
-
-Need help? See the following resources:
-- Spago documentation: https://github.com/purescript/spago
-- Dhall language tour: https://docs.dhall-lang.org/tutorials/Language-Tour.html
-
-When creating a new Spago project, you can use
-`spago init --no-comments` or `spago init -C`
-to generate this file without the comments in this block.
--}
 { name = "my-nextjs-project"
 , dependencies =
   [ "console"