External Media / AI Image Generator: update button style (#39985)

* External Media / AI Image Generator: update button style

See #39934

Let's change the overall styles of the 2 buttons ("Select Image" and "Generate with AI"):

- the buttons should be 40px high, like in Core. See WordPress/gutenberg#46741
- the buttons should use the secondary style variant by default, to match the "Insert from URL" button.
- the buttons should take the full width of the placeholder on mobile devices.

* Ensure the buttons look good in narrow parent blocks

See Automattic/jetpack#39985 (comment)

* Simplify styling of the main wrapper

The wrapper itself is still necessary unfortunately.

Co-authored-by: Filipe Varela <keoshi@keoshi.com>
Co-authored-by: jasmussen <joen@git.wordpress.org>

* Add logic to change button size based on wp & Gutenberg versions

The other buttons in the Image block were changed in this PR:
WordPress/gutenberg#65361

This was released with Gutenberg 19.4, and will be included in WP version 6.7.

Let's consequently set our own buttons to match.

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Co-authored-by: Filipe Varela <keoshi@keoshi.com>
Co-authored-by: jasmussen <joen@git.wordpress.org>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/11660975525

Upstream-Ref: Automattic/jetpack@746d1fc