Bump CI actions to Node.js 24 versions

- actions/upload-artifact v4 -> v6
- actions/download-artifact v4 -> v6
- docker/setup-qemu-action v3 -> v4

v5 of the artifact actions had preliminary Node 24 support but still
defaulted to Node 20. v6 is the actual Node 24 runtime switch.

Author: j-256

.github/workflows/ci.yml

@@ -66,7 +66,7 @@ jobs:
 
       - name: Set up QEMU
         if: matrix.platform == 'linux/arm64'
-        uses: docker/setup-qemu-action@v3
+        uses: docker/setup-qemu-action@v4
 
       - name: Build (Linux static)
         if: runner.os == 'Linux'
@@ -90,7 +90,7 @@ jobs:
           ./bin/git-crypt --version
           file ./bin/git-crypt
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: git-crypt-${{ matrix.target }}
           path: bin/git-crypt
@@ -125,7 +125,7 @@ jobs:
             exit 1
           fi
 
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@v6
         with:
           pattern: git-crypt-*
           path: artifacts/