Update GitHub Actions CI

The following updates are performed:
* update actions/checkout to v4
* replace unmaintained actions-rs/toolchain by dtolnay/rust-toolchain
* replace unmaintained actions-rs/cargo by direct invocation of cargo

.github/workflows/deploy-docs.yml

@@ -26,17 +26,12 @@ jobs:
   build_docs:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Setup Pages
         uses: actions/configure-pages@v4
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
+      - uses: dtolnay/rust-toolchain@stable
       - name: Build docs
-        uses: actions-rs/cargo@v1
-        with:
-          command: doc
-          args: --no-deps
+        run: cargo doc --no-deps
       - name: Fix docs
         run: |
           rm -rf ./docs