Shift to new combination Quidel signal - prerequisite deletion support

[krivard]

The new threshold handling in the combined quidel indicator will remove records for some regions. The API data versioning will still report those regions unless we implement deletion encoding first (issue reference TBA).

Activating the new combined Quidel signal should come with a documentation update.

[jingjtang]

@krivard What will happen without the deletion support if the value exists for location A in the data reported for date D with issue date E, but the value does not exist for the same location reported for the same date but a different issue date E' ?

[krivard]

Assuming E' > E, the API will report the value from issue E.