Switch of `v2-staging` branch to master lost changelog info

Releases for `v1.8.0`, `v1.8.1`, and `v.1.9.0` no longer have entries in [`CHANGELOG.md`](https://github.com/googleapis/python-firestore/blob/master/CHANGELOG.md).

Fix would be to copy them in from the `v1` branch.