Upgrade automerge to version 2 of google-cloud-secret-manager

[balopat]

1. We should write some tests for automerge.
2. We should upgrade the secret manager

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It looks like there's a version 2 of the google-cloud-secret-manager library available, and a migration guide: https://googleapis.dev/python/secretmanager/latest/UPGRADING.html.

Originally posted by @maffoo in #3770 (comment)

[ayshiff]

I'd be happy to help with this.

[mhucka]

Closing because this has been resolved by #7214.