updated to new key for update timestamp

[aysim319]

Description

update update timestamp key value from rowsUpdatedAt to viewLastModified to reflect the changes from cdc data metadata api response

Associated Issue(s)

• Addresses update checking updates for nhsn #2152

[melange396]

should also change:

covidcast-indicators/nhsn/tests/conftest.py

Line 75 in 9e85766

mock_get_metadata.return_value = {"rowsUpdatedAt": time.time()}

[melange396]

and:

covidcast-indicators/nhsn/tests/test_pull.py

Line 89 in 9e85766

mock_client.get_metadata.return_value = {"rowsUpdatedAt": now}