Deprecate configuring Gitaly nodes in virtual storage's config root

What does this MR do?

Historically, a virtual storage's Gitaly nodes have been configured root of the virtual storage's config object. This didn't work well, as the configuration format did not match the structure Praefect itself was using and made it awkward to add virtual storage scoped configuration keys. Since then, we've modified the configuration format in Omnibus to also put the nodes under the 'nodes' key and included fallback code to retain backwards compatibility. In 15.0, we have the chance to remove the fallback code. To prepare for that, this commit adds logs a deprecation message if the there are some nodes configured in the root of the virtual storage's config rather than under the 'nodes' key.

Related issues

#5981 (closed)

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