Log all index settings updates at INFO level

Most updates to index settings are handled by a settings update consumer registered via IndexScopedSettings#addSettingsUpdateConsumer. In contrast, the settings defined in IndexMetaData (index.blocks.*, index.priority, etc.) are consumed directly from the Settings object instead. Changes to index settings that are consumed via a settings update consumer result in an INFO-level log message (see e.g. #49969). Changes to directly-consumed settings do not automatically result in any log messages, and we explicitly handle some special cases like index.number_of_replicas elsewhere.

I think this is confusing since from a user's perspective there's no way to distinguish these two classes of index settings. A user seeing that some settings updates are logged may wrongly infer that the lack of logging of other settings updates means that those other settings were not updated.

Should we make this consistent? Consistency is good, but noisier logs are bad.