Redirect old /hello-world to /blog/hello-world URL

functions.php

<?php

add_action('template_redirect', 'redirect_old_post_urls');
function redirect_old_post_urls() {
    if (is_404()) {
        global $wp;
        $request = $wp->request;
        
        $post = get_page_by_path($request, OBJECT, 'post');
        
        if ($post) {
            wp_redirect(home_url("/blog/{$request}/"), 301);
            exit;
        }
    }
}