Decide what to do with description property in MediaSource plugin

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(description)

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

Steps to reproduce

Proposed resolution

If description is not used then deprecate it

Remaining tasks

1) Determine if description is used.

User interface changes

API changes

Data model changes

Release notes snippet