Pull deprecated status from config.schema.json, make warnings generic #1680

Is your feature request related to a problem? Please describe.
In #1626, we added deprecation warnings for legacy config fields such as sslKeyPemPath, sslCertPemPath and proxyUrl. However, the checks are baked into src/config/deprecatedFields.ts. We can make this cleaner and more extensible by picking up deprecated fields from config.schema.json and keeping it as the source-of-truth.

Describe the solution you'd like
Fix getDeprecatedConfigWarnings() to pull the deprecated status from schema.config.json and use that to generate the warnings. Perhaps we could add x-deprecated-replacement to the schema so users how to replace the deprecated fields.

Additional context
Mentioned in #1629 and #1665, original PR #1626, related issue #1545