Upgrade GitHub Actions to latest versions (#336)

## Summary

Upgrade GitHub Actions to their latest versions for improved features,
bug fixes, and security updates.

## Changes

| Action | Old Version(s) | New Version | Release | Files |
|--------|---------------|-------------|---------|-------|
| `docker/build-push-action` |
[`v5`](https://github.com/docker/build-push-action/releases/tag/v5) |
[`v6`](https://github.com/docker/build-push-action/releases/tag/v6) |
[Release](https://github.com/docker/build-push-action/releases/tag/v6) |
build_kernel.yaml, docker-build-push.yaml |

## Why upgrade?

Keeping GitHub Actions up to date ensures:
- **Security**: Latest security patches and fixes
- **Features**: Access to new functionality and improvements
- **Compatibility**: Better support for current GitHub features
- **Performance**: Optimizations and efficiency improvements

### Security Note

Actions that were previously pinned to commit SHAs remain pinned to SHAs
(updated to the latest release SHA) to maintain the security benefits of
immutable references.

### Testing

These changes only affect CI/CD workflow configurations and should not
impact application functionality. The workflows should be tested by
running them on a branch before merging.

Signed-off-by: Salman Muin Kayser Chishti <13schishti@gmail.com>

Author: salmanmkc

.github/workflows/build_kernel.yaml

@@ -92,7 +92,7 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Build Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: tests/Dockerfile.test-kernel

.github/workflows/docker-build-push.yaml

@@ -53,7 +53,7 @@ jobs:
             type=semver,prefix=user-,pattern={{major}}.{{minor}}
 
       - name: Build and push Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./dockerfiles/Dockerfile.user
@@ -104,7 +104,7 @@ jobs:
             type=semver,pattern={{major}}.{{minor}}
 
       - name: Build and push Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./dockerfiles/Dockerfile