Make updatecheck.py less prone to triggering API rate limits

[str4d]

qa/zcash/updatecheck.py checks that our C++ dependencies are either up-to-date or postponed. Most of the dependencies are hosted on GitHub, and the script uses the GitHub API to fetch version information for those dependencies. However, the way it is currently implemented causes many separate requests to be made, which can quickly cause IP address rate limits to be exceeded. This is particularly problematic when trying to test that a new dependency is being checked correctly, or an update has been correctly postponed.

The script should be reworked to reduce the number of GitHub API calls it makes. This might require switching to the GraphQL API, in which case the process for running it would need to be updated / documented, and the release script and process similarly updated.