Deprecate ToStringTrait

Problem/Motivation

The trait was used for workaround of PHP bug which is fixed since 7.4 https://3v4l.org/5ZJ2i
It has only 2 usages and DTP already has issue #2828835: Drupal DateTime have no render to json_encode →

Steps to reproduce

Follow-up from #3465827-5: Stop passing E_USER_ERROR to trigger_error() on PHP 8.4 →

Proposed resolution

- deprecate the trait or wait for 12.x
- fix usage

Remaining tasks

- patch
- review
- commit

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet