fix(cd): Update cd process to push manifests instead of using buildx

[adityasadalage]

Problem

We are pushing amd and arm images using docker's buildx plugin which is nice and clean. However, we use manifests to push dgraph and dgraph:standalone images in the core repo which gives us much better control over the image build/push process. We should standardize this process across releases in other key repos like lambda as we build multi-arch support.

Solution

Converting the cd-dgraph-lambda process to build and push using manifests

[github-actions[bot]]

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