Update use of download-artifact to v4

Reading through the migration guides [for
download-artifact@v4](https://github.com/actions/download-artifact/blob/main/docs/MIGRATION.md)
and [for
upload-artifact@v4](https://github.com/actions/upload-artifact/blob/main/docs/MIGRATION.md),
I don’t think we’re currently using any of the features of v3 that have
been removed in v4.

So I think just updating the version numbers should be sufficient.

README.md

@@ -1,4 +1,4 @@
-# @guardian/actions-static-site V2
+# @guardian/actions-static-site V3
 
 Action to provision and serve a static site with Google Auth and a custom
 domain.
@@ -33,12 +33,12 @@ jobs:
       # ... (Build your static site.)
 
       # Then upload it as an artifact
-      - uses: actions/upload-artifact@v3
+      - uses: actions/upload-artifact@v4
         with:
           path: my-site
 
       # Then invoke this action (replacing app and domain)
-      - uses: guardian/actions-static-site@v2
+      - uses: guardian/actions-static-site@v3
         with:
           app: example
           domain: example.gutools.co.uk

action.yaml

@@ -35,7 +35,7 @@ runs:
         INPUT_DRYRUN: ${{ inputs.dryRun}}
         INPUT_ACTIONS_RUNTIME_TOKEN: ${ github.token }
 
-    - uses: actions/download-artifact@v3
+    - uses: actions/download-artifact@v4
       with:
         name: ${{ inputs.artifact }}
         path: site