ci: 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/build-publish-dispatch.yml

@@ -35,7 +35,7 @@ jobs:
 
       - name: Get Docker tags
         id: docker_meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: |
             nginxproxy/nginx-proxy

.github/workflows/build-publish.yml

@@ -46,7 +46,7 @@ jobs:
 
       - name: Get Docker tags
         id: docker_meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: |
             ghcr.io/nginx-proxy/nginx-proxy