kubernetes: drop older DeploymentsConfig support
Removes transitional support for the T299648-style DeploymentsConfig format, now that we've migrated to the new format added for T370934.
As a result, the 'releases' field is now required for any configured namespace. This is now enforced in production puppet after [0].
[0] https://gerrit.wikimedia.org/r/1077483
Bug: T370934