github-actions: bump docker/metadata-action from 5 to 6 (#776)

Bumps [docker/metadata-action](https://github.com/docker/metadata-action) from 5 to 6.
- [Release notes](https://github.com/docker/metadata-action/releases)
- [Commits](docker/metadata-action@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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/docker.yml

@@ -68,7 +68,7 @@ jobs:
 
       - name: Docker meta
         id: meta
-        uses: docker/metadata-action@c1e51972afc2121e065aed6d45c65596fe445f3f # v5.8.0
+        uses: docker/metadata-action@030e881283bb7a6894de51c315a6bfe6a94e05cf # v6.0.0
         with:
           images: |
             openthread/otns-${{ matrix.image }}
@@ -145,7 +145,7 @@ jobs:
 
       - name: Docker meta
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: |
             openthread/otns-${{ matrix.image }}