Intent to deprecate

[devoncarew]

As an update for consumers of this package, we intend to deprecate package:usage. #185 has details and discussion, but briefly:

• this package uses the older Universal Analytics protocol to record hits to Google Analytics
• Universal Analytics properties will stop processing new hits on July 1, 2023
• this library does not support the newer GA4 protocol
• we (the Dart team) originally built this package for our own use, but we're no longer consuming it

Clients who want to record hits to Google Analytics could:

• for web clients, wrapping the gtags.js library is a good solution
• for native clients, sending hits to the new protocol is not much more difficult than sending HTTP POSTs to the right endpoint

See also #185 other potential remediations.