fix vm-informant dbname: "neondb" -> "postgres"

[sharnoff]

Describe your changes

Changes the vm-informant's postgres connection string's dbname from "neondb" (which sometimes doesn't exist) to "postgres" (which hopefully should exist more often?).

Currently there are a handful of VMs in prod that aren't working with autoscaling because they don't have the "neondb" database.

The vm-informant doesn't require any database in particular; it's just connecting as cloud_admin to be able to adjust the file cache settings.

[github-actions]

Test results for 52810e4:

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debug build: 214 tests run: 204 passed, 0 failed, 10 (full report)

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release build: 214 tests run: 204 passed, 0 failed, 10 (full report)

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[sharnoff]

Double-checked that the API does not allow you to delete the "postgres" database, even though it does let you delete "neondb".