Don't fail the release when the downloads server is unreachable (#475)

The "Release the book and website" workflow has been failing on every
push to main. The book and website build fine, but the deploy step
("Copy book to downloads server") can't reach the host in SSH_HOST:

    ssh: connect to host *** port 22: Network unreachable
    rsync error: unexplained error (code 255)

That's an infrastructure issue (server down / moved / firewalled), not a
problem with the book. Mark both rsync deploy steps continue-on-error so
a temporarily unreachable downloads server no longer fails the whole
release - the book/website artifacts are still uploaded to GitHub.

Also bump the deprecated actions flagged in the run annotations:
- burnett01/rsync-deployments 7.0.2 -> v8 (inputs unchanged)
- actions/checkout v3 -> v4 (Node 20 is being forced to Node 24)

Author: hendricius

.github/workflows/release-book-website.yml

@@ -16,7 +16,7 @@ jobs:
       packages: write
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Log in to the Container registry
         uses: docker/login-action@65b78e6e13532edd9afa3aa52ac7964289d1a9c1
@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Set up git repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Print dependency versions
         uses: addnab/docker-run-action@v3
         with:
@@ -66,7 +66,11 @@ jobs:
             cd /app/book
             make -j -O bake
       - name: Copy book to downloads server
-        uses: burnett01/rsync-deployments@7.0.2
+        # Best-effort: a temporarily unreachable downloads server must not
+        # fail the whole release. Book/website artifacts are still uploaded
+        # to GitHub below.
+        continue-on-error: true
+        uses: burnett01/rsync-deployments@v8
         with:
           switches: "-avzr"
           remote_host: ${{ secrets.SSH_HOST }}
@@ -83,7 +87,9 @@ jobs:
             book/book_serif/book.pdf
             book/book-epub/book.epub
       - name: Copy website to downloads server
-        uses: burnett01/rsync-deployments@7.0.2
+        # Best-effort, same rationale as the book deploy above.
+        continue-on-error: true
+        uses: burnett01/rsync-deployments@v8
         with:
           switches: "-avzr --delete"
           remote_host: ${{ secrets.SSH_HOST }}