Description

Instead of writing custom HTML for the admin notice, we should call the wp_admin_notice() function. https://developer.wordpress.org/reference/functions/wp_admin_notice/

Step-by-step reproduction instructions

1. Go to https://github.com/WordPress/gutenberg/blob/trunk/docs/how-to-guides/notices/README.md#notices-in-the-classic-editor.
2. There is custom HTML code for the admin notice.

Screenshots, screen recording, code snippet

Environment info

No response

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

• Yes

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

• Yes

Please confirm which theme type you used for testing.

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