Update eloquent-mutators.md

[faiare]

This PR clarifies the behavior of timestamp columns in relation to date/datetime casting and timezone handling in Laravel.

Key points:

1. This update is related to [10.x] Explain UTC behaviour when serialising dates #9287 and Serializing datetimes with custom format won't use the Carbon instance inner timezone. framework#49899.

2. While the referenced issues state that dates are always displayed in UTC regardless of custom formatting, this is not entirely accurate. For non-timestamp columns, Carbon instances are created based on the app.timezone configuration and formatted in local time, as per the original documentation.

3. However, for timestamp columns (created_at, updated_at), the timezone setting is lost during pre-casting processing (serializeDate, Date::parse, format methods), resulting in consistent UTC formatting.

4. This discrepancy can lead to confusion for developers who expect consistent behavior across all date columns with similar cast settings.

5. It's important to note that following the documentation's recommendation to keep app.timezone set to the default UTC value would prevent these inconsistencies.

6. Given the potential impact on many applications, we believe updating the documentation is preferable to modifying this core behavior.