chore(main): release 2.8.20-SNAPSHOT (#965)

:robot: I have created a release *beep* *boop*
---


### Updating meta-information for bleeding-edge SNAPSHOT release.

---
This PR was generated with [Release Please](https://togithub.com/googleapis/release-please). See [documentation](https://togithub.com/googleapis/release-please#release-please).
6 files changed
tree: 398a8a59518e2a7ae6dbddd240d2fe7747e3a789
  1. java-core/