Video Block: Add accessibility name to core/video block using a caption

[shubhtoy]

Description

The following issue was raised on trac: #63519 by @rlautan

The core/video block that uses a caption has no accesibility name. Adding one as a aria-label or a aria-labbeledby attribute will enhance the accessibility for the block.

Step-by-step reproduction instructions

1. Create a new page/post
2. Add a core/video block
3. Add media and caption to it
4. Navigate to the frontend, and using a screenreader try to focus on the video.
5. Verify that the screenreader announces the video but not the caption

Screenshots, screen recording, code snippet

https://github.com/user-attachments/assets/90045664-b899-4b55-b8e0-cbdbb2a8791d

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• [x] Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• [x] Yes

Please confirm which theme type you used for testing.

• [ ] Block
• [ ] Classic
• [ ] Hybrid (e.g. classic with theme.json)
• [ ] Not sure