[CT-1301] Behavior change: `full_refresh: true` config should respect `--full-refresh, -f` flag

https://docs.getdbt.com/reference/resource-configs/full_refresh

### Current behavior
When configured, `full_refresh: true|false` means: "always/never full-refresh, and always ignore the --full-refresh flag"

Added way back in https://github.com/dbt-labs/dbt-core/pull/2438 / v0.18

### Proposed change
- `full_refresh: true` should mean "always respect the --full-refresh flag"
- `full_refresh: false` should mean "never full refresh, under any circumstances"

That's trickier for intuition, and harder to explain, but definitely more desirable in terms of the behavior it enables. I'm not sure why you'd always want to full-refresh an `incremental` model; at that point it's really just a `table` materialization.

If this is set at the project level:
```yml
models:
+full_refresh: false
```
It's not possible to override that setting for one particular model ("actually I want this model to respect the `--full-refresh` flag"). This doesn't work as I'd hoped it might:
```sql
{{ config(materialized = 'incremental', full_refresh = none) }}
```

### Is this a breaking change?

Only for people who currently make use of `full_refresh: true`. Again, I'm not actually sure what the use case for doing that would be!