Terraform destroy calls CustomizeDiff though documentation suggests it shouldn't be

### SDK version

```
v2.24.1
```

### Relevant provider source code

https://github.com/prashantv/terraform-customizediff-on-destroy-repro

### Terraform Configuration Files

Config doesn't matter, as it seems to always happen. However, the above repo does contain a repro with config.

### Debug Output

https://gist.github.com/prashantv/5e00eb5e6dc9215c280d7cb30a68cf26

### Expected Behavior
CustomizeDiff shouldn't be called, as documented in the [SDK docs](https://pkg.go.dev/github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema#Resource).
> // * Destroy: No runs (standard diff logic is skipped on destroy diffs)

If it is called, there should be some way for the provider to know that this is a destroy call.

### Actual Behavior

CustomizeDiff is called.

### Steps to Reproduce

See https://github.com/prashantv/terraform-customizediff-on-destroy-repro

Start with the linked terraform-customizediff-on-destroy-repro repository and its configuration, then compare the observed behavior with the CustomizeDiff documentation and the linked debug output. Done means determining why CustomizeDiff runs during destroy and either preventing that call or exposing whether the operation is a destroy, with the documented behavior and reproduction covered.

Written by the indexing model from the issue text.