Surface deprecation warnings when loading the project model

### What needs to get done

When loading the project model, surface deprecation warnings as warnings to the user.

### Why it needs to get done

This allows us to [mark fields as deprecated](https://docs.pydantic.dev/latest/concepts/fields/#deprecated-fields) without [ugly workarounds](https://github.com/canonical/charmcraft/pull/1828)

Start by locating the project model loading path and how Pydantic validation warnings are currently handled. Reproduce loading a model with a deprecated field, then verify that its deprecation warning is surfaced to the user without requiring a workaround.

Written by the indexing model from the issue text.