config: Extend coverage of Gitaly dependencies
Extend coverage of Gitaly's dependencies to include some of the more important dependencies:
- Golang's own `golang.org/x/` packages for that aren't part of the
main Go tree.
- Packages required to connect to the Postgres database.
- gRPC-related packages required to implement gRPC services and
clients.
For the time being we still only monitor a subset of dependencies via Renovate to keep the initial churn of dependency updates low.