Describe the bug

When I add related meetings to a conference speaker, they don't get saved

To Reproduce
Steps to reproduce the behavior:

1. Sign in as admin
2. Go to admin dashboard, conference, speakers, create a new speaker
3. Fill in the required fields with any participant type
4. Select a related meeting
5. Save the form
6. Edit the same speaker
7. See that it doesn't save the related meetings

Expected behavior
To get it saved when editing or seeing it in the frontend

Extra data (please complete the following information):

• Decidim Version: v0.28.0.dev
• Decidim installation: Nightly

Additional context

We need to add the required field to the form