Search: Improve messaging for the overlay close button on Customberg

### Impacted plugin

Jetpack

### Steps to Reproduce

1. On a site with a Jetpack Search subscription with Instant Search enabled, navigate to Customberg (`/wp-admin/admin.php?page=jetpack-search-configure`).
2. Click on the `X` adjacent to the search box in the Instant Search application:

### A clear and concise description of what you expected to happen.

`X` should either:
- indicate it's not an interactable element
- take the user back to the last page in the browser history stack

### What actually happened

`X` is clickable, but doesn't do anything besides drawing an outline on the button.

### Other information

_No response_

### Operating System

_No response_

### OS Version

_No response_

### Browser

Chrome/Chromium

### Browser Version(s)

_No response_

Start at the Customberg entry point in /wp-admin/admin.php?page=jetpack-search-configure and reproduce the Instant Search overlay behavior in Chrome/Chromium. Inspect the X adjacent to the search box and clarify whether the intended outcome is a non-interactable control or navigation to the previous browser-history page; done means the selected behavior is implemented and the outline-only action no longer occurs.

Written by the indexing model from the issue text.