Decide what to do with thumbnail_title_metadata_attribute

Problem/Motivation

This is a followup from 📌 Convert MediaSource plugin discovery to attributes Active based on this comment by alexpott.

In the other issue alexpott could not find any usage of \Drupal\media\Attribute\MediaSource::__construct(thumbnail_title_metadata_attribute)

This issues is to confirm that it is not used and then deprecate it..

Steps to reproduce

Proposed resolution

If thumbnail_title_metadata_attribute is not used then deprecate it

Remaining tasks

1) Determine if thumbnail_title_metadata_attribute is used.

User interface changes

API changes

Data model changes

Release notes snippet