build(deps): bump docker/metadata-action from 5 to 6

Bumps [docker/metadata-action](https://github.com/docker/metadata-action) from 5 to 6.
- [Release notes](https://github.com/docker/metadata-action/releases)
- [Commits](https://github.com/docker/metadata-action/compare/v5...v6)

---
updated-dependencies:
- dependency-name: docker/metadata-action
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/docker.yml

@@ -24,7 +24,7 @@ jobs:
 
       - name: Generate Alpine Docker tags
         id: docker_alpine_tags
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: |
             docker.io/${{ github.repository_owner }}/miniflux
@@ -37,7 +37,7 @@ jobs:
 
       - name: Generate Distroless Docker tags
         id: docker_distroless_tags
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: |
             docker.io/${{ github.repository_owner }}/miniflux