Drop support for deprecated config properties

[tatomyr]

Is your feature request related to a problem? Please describe.

Currently Redocly config supports a bunch of fields that have been deprecated for a while. This results in config being hard to maintain.

Describe the solution you'd like

Remove support for deprecated options: 'features.openapi', 'features.mockServer', 'styleguide', 'lint', 'apiDefinitions', and 'referenceDocs' and streamline the config resolving code.

Describe alternatives you've considered

Continue supporting the stale syntax with warnings.

Additional context

We can combine this with other removals like this one.