Replace secrets with the new ones (#1564)

In the monorepo, secrets used in the various previous repos have been
renamed into:
- `CUMULUS_DOCKERHUB_USERNAME`
- `CUMULUS_DOCKERHUB_TOKEN`
- `POLKADOT_DOCKERHUB_USERNAME`
- `POLKADOT_DOCKERHUB_TOKEN`

This PR makes those changes and remove one of the GHW that has now been
updated for the monorepo.

.github/workflows/release-50_publish-docker.yml

@@ -198,8 +198,8 @@ jobs:
       - name: Login to Dockerhub
         uses: docker/login-action@v2
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
+          username: ${{ secrets.CUMULUS_DOCKERHUB_USERNAME }}
+          password: ${{ secrets.CUMULUS_DOCKERHUB_TOKEN }}
 
       - name: Push Container image for ${{ env.BINARY }}
         id: docker_push
@@ -255,8 +255,8 @@ jobs:
       - name: Login to Docker Hub
         uses: docker/login-action@343f7c4344506bcbf9b4de18042ae17996df046d # v3.0.0
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
+          username: ${{ secrets.POLKADOT_DOCKERHUB_USERNAME }}
+          password: ${{ secrets.POLKADOT_DOCKERHUB_TOKEN }}
 
       - name: Fetch values
         id: fetch-data