Update upload/download artifact action.

Needs to be done in one go, so we can't use the dependabot PRs.

.github/workflows/ci.yml

@@ -44,7 +44,7 @@ jobs:
       # And lastly, we upload the generated build/ folder as artifact so we can
       # use it in later steps.
       - name: Upload build and .svelte-kit folder
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: build
           if-no-files-found: error
@@ -85,7 +85,7 @@ jobs:
       - uses: actions/checkout@v3
 
       # Download the build from the previous step.
-      - uses: actions/download-artifact@v3
+      - uses: actions/download-artifact@v4
         with:
           name: build
           path: build
@@ -127,7 +127,7 @@ jobs:
     steps:
       # No need to checkout the project, since all we need is to download the
       # build artifact from the build step.
-      - uses: actions/download-artifact@v3
+      - uses: actions/download-artifact@v4
         with:
           name: build
           path: build