Remove redundant key/value pair from the payload of DORA metrics API

What does this MR do?

This MR simply removes the { 'date' => 'value' } structured entry from the DORA metrics API. This entry was mistakenly added and already replaced by the new { "date" => "...", "value" => ... } format in the previous milestone.

This is technically a breaking change, however, we don't expect any magnitude by this change as the API is relatively new which was just introduced two months ago.

Related #325931 (closed)

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