[9.x] Fix updated_at timestamp not updating when field is read-only

[duncanmcclean]

This pull request fixes an issue where the updated_at timestamp wasn't being updated when it was configured as a read_only field in the blueprint.

This was happening because read_only fields are still saved, so the value was run through the fieldtype's process() method and set on the model via setAttribute. This made the updated_at attribute dirty in the model's attributes array, which prevented Laravel's updateTimestamps() from auto-updating the timestamp (it skips columns that are already dirty), causing the stale value to be saved.

This PR fixes it by skipping setAttribute for timestamp columns (updated_at/created_at) when the field is read_only and the model uses timestamps, allowing Laravel's automatic timestamp handling to work as expected.

Fixes #805

[github-actions]

Released as part of v9.4.4.