Revert image manifest changes

[calebamiles]

Reverting changes made to the release tooling to support image manifests as we were unable to get the
changes working in time for the 1.12.0. We'll try again for 1.12.2 at the earliest.

[dims]

cc @mkumatag - fyi, we could not switch on experimental in the docker CLI. from last attempt it seems like anago or gcbmgr may be overwriting if we set the value in config.json. that's the last theory we had

[tpepper]

The way the reverts show in the history (2x merge revert and 2x commit revert, to revert 3x merges and 4x commits) looks odd, but it’s reverting all changes back to 7e27e08.

/lgtm

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[calebamiles]

Yeah that's the beauty of merge commits, @tpepper 😭

[mkumatag]

I think we need to spend more time on testing.. I don't have a exact environment, I'll spend some time mimicking this env later.