ci: bump GitHub Actions to v5 for Node 24 compatibility

actions/checkout, actions/cache, actions/upload-artifact, and
actions/download-artifact bumped from v4 to v5 across both
ci.yml and release.yml. Pre-empts the 2026-06-02 Node 20
deprecation deadline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable
@@ -33,7 +33,7 @@ jobs:
             libxkbcommon-dev
 
       - name: Cache cargo registry and build artifacts
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: |
             ~/.cargo/registry
@@ -59,7 +59,7 @@ jobs:
     needs: test
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable
@@ -74,7 +74,7 @@ jobs:
             libxkbcommon-dev
 
       - name: Cache cargo registry and build artifacts
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: |
             ~/.cargo/registry