[4.x] Unify image:field_image_caption to image:field_caption

✨ Add "Licence / Source" field to all media entities Active is related, but that should be a separate field.

Added and configured the new field in our preset @anybody, but I can't delete the old field_image_caption because the fields are locked, and I don't want to perform CEX/CIM at this state.

Drush or Devel PHP?

@thomas.frobieter we had field_image_caption existing for slides but field_caption is not present on slides.

Now that we'll remove field_image_caption what's the plan for slides?

Now that we'll remove field_image_caption what's the plan for slides?

Since 4.x is backwards compatible, we might need a database update, wouldn't we?

Otherwise, all existing media slide overlay texts are gone.

@Grevil: Can you implement the update hook by time?

See #10. The field only existed in slides. The update hook should also remove the field programmatically then. In our Vorlage I already removed it manually to proceed. So should only execute if it's present, otherwise all good. Our Vorlage had no important existing values for that.

@thomas.frobieter: We must be careful, for Slides the field_image_caption field is labeled "Overlay-Text"!
Should we keep that label?

@Grevil: The update script is more important than I had thought before. As @thomas.frobieter wrote, we'll need this for upgradability from 3.x to 4.x

The update script needs to perform the following tasks for the affected media types:

1. Check if field_caption already exists. If no, create it as a copy of field_image_caption. If it already exists, skip this steps (this is the case in our preset). The new field should have the same weight as the old field in form and display. [field / config migration]
2. Loop all entities of the affected media types and check if both field_image_caption and field_caption exist on the media type. If yes, copy the values from field_image_caption to field_caption [data migration]
3. If field_image_caption exists, remove it. [cleanup]

Important notes:

• In our Vorlage, the steps were already taken manually to speed things up and not block us. The update hook should not break things there.
• The newly added field_caption in documents doesn't have to be considered, but we'll need to newly create it programmatically here: ✨ [4.x] Add field_caption for Documents Needs work , I think by importing the config and setting the field weights in display programmatically afterwards. See the other issue, you may want to do that after this issue.

@thomas.frobieter: We must be careful, for Slides the field_image_caption field is labeled "Overlay-Text"!
Should we keep that label?

We should, because the other fields are also named like this and "Caption" would be kind of misleading here.

Re #19 done!