Feature Request: convert search widget to block widget

### Impacted plugin

Jetpack, Search

### What

Remove Jetpack search widget from legacy widgets and make it support block themes

### How

Docs: https://developer.wordpress.org/block-editor/how-to-guides/widgets/legacy-widget-block/

It's surprisingly easy to do so. See example: https://github.com/Automattic/jetpack/pull/21050

Start with the Jetpack Search widget and read the linked WordPress legacy-widget block guide, then compare the example pull request mentioned in the issue. Done means the Search widget is removed from legacy widgets and works with block themes; the issue names no specific files or tests, so locating the implementation and verification steps will require repository research.

Written by the indexing model from the issue text.