README contains broken links and inaccurate information

[meredithslota]

The .readme-partials.yaml file is out-of-date and is creating broken links in the main README file: https://github.com/googleapis/java-datastore/blob/main/.readme-partials.yaml

Specifically, the "Testing" section points to a broken link, as does the "Complete source code" section. There may be other errors as well.