[Serve] Deprecate outdated REST API settings

[shrekris-anyscale]

Why are these changes needed?

Since #25691 introduced end-to-end REST API and CLI support for the new Serve config format, the outdated format can be phased out. This change removes now-outdated fields from the config, including:

• (Deployment-level) import_path
• init_args
• init_kwargs

Note that these settings can still be configured through the Python API. This change affects only Serve's REST API and CLI.

To further support this change, outdated functionality from Application is also removed, including the following methods:

• to_dict()/from_dict()
• to_yaml()/from_yaml()

Their functionality is moved to the few dependent pieces of code, and their implementations are removed.

Related issue number

N/A

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
    • This change relies on existing unit tests. Outdated tests are updated or removed.