ci: bump checkout to v5 and setup-uv to v6 (Node 20 deprecation)

GitHub is force-running Node 20 actions on Node 24 and warning about it.
Both bumped to their Node 24 majors; no behaviour change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ahrzb

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       # The package is a pyo3/maturin extension, so CI needs a Rust toolchain
       # to build it during `uv sync`.
@@ -24,7 +24,7 @@ jobs:
         uses: Swatinem/rust-cache@v2
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v5
+        uses: astral-sh/setup-uv@v6
         with:
           enable-cache: true