[5.x] Make alt text from image metadata optional

[redburn]

Since #18744 (requested here), Craft fills an empty alt on upload from the file's metadata. This metadata may contain real alt text, but in many cases photographers also use this metadata for credit lines and shoot details. That now ends up as the alt text, so screen readers may announce the date, the place and the photographer instead of what the photo shows.

I understand using the caption was a deliberate choice, and it helps where captions are written as descriptions. In our case alt texts are generated with AI, but if alt text is already written by Craft from the metadata then that's not overwritten (we only generate when alt is empty).

Would you consider one of these?

• a general config setting to switch the metadata alt off, or to choose the sources (e.g. only AltTextAccessibility)
• only using AltTextAccessibility by default, since IPTC introduced it precisely so alt text and caption are kept apart