Gallery block: Strip HTML from caption before using as alt attribute.

[ockham]

Describe the bug
Per #26082, in the gallery block, for an image without an alt text specified, we're using the image's caption as a fallback. However, the caption can contain HTML (e.g. a link, i.e. an <a /> tag) that we should strip from it when using it as alt text.

To reproduce
Steps to reproduce the behavior:

1. Insert a gallery block.
2. Add a few images.
3. Add a caption to one of the images.
4. Highlight part of the caption and add a link (e.g. to wordpress.org).
5. Publish the post, and view it one the frontend.
6. Inspect the page source.
7. Verify that the image's alt attribute includes the HTML (<a /> element).

Expected behavior
HTML should be removed from the caption when used as alt attribute.

Screenshots
N/A

Editor version (please complete the following information):

• WordPress version: 5.5
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? Gutenberg plugin v9.2.2 installed.
• If the Gutenberg plugin is installed, which version is it? [e.g., 7.6]

Brought up here. When used with other plugins, there can be issued with incorrectly mangled HTML in attributes, see e.g. Automattic/wp-calypso#46813.

cc/ @ZebulanStanphill

[ockham]

Let's also add test coverage when providing a fix 🙂

[ockham]

Aaand I guess we'll need a migration, since this will be changing the block content (as saved by save.js).

[ockham]

We can probably use __unstableStripHTML() from @wordpress/dom here (need to figure out why it's marked as unstable).