feat(tracking): include the web-server family in server data

Adds $_SERVER['SERVER_SOFTWARE'] (e.g. "Apache/2.4.62", "nginx/1.24.0",
"Microsoft-IIS/10.0") to the tracking payload's server collection so we
can size what proportion of installs run on each server family. Output
key: server.ServerSoftware. Empty string when the SAPI does not populate
SERVER_SOFTWARE (rare; e.g. some CLI contexts).

Refs Yoast/wordpress-seo-premium#4968.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: enricobattocchi

admin/tracking/class-tracking-server-data.php

@@ -36,10 +36,11 @@ protected function get_server_data() {
 			$server_data['Hostname'] = gethostbyaddr( $ipaddress );
 		}
 
-		$server_data['os']            = function_exists( 'php_uname' ) ? php_uname() : PHP_OS;
-		$server_data['PhpVersion']    = PHP_VERSION;
-		$server_data['CurlVersion']   = $this->get_curl_info();
-		$server_data['PhpExtensions'] = $this->get_php_extensions();
+		$server_data['os']             = function_exists( 'php_uname' ) ? php_uname() : PHP_OS;
+		$server_data['PhpVersion']     = PHP_VERSION;
+		$server_data['CurlVersion']    = $this->get_curl_info();
+		$server_data['PhpExtensions']  = $this->get_php_extensions();
+		$server_data['ServerSoftware'] = isset( $_SERVER['SERVER_SOFTWARE'] ) ? sanitize_text_field( wp_unslash( $_SERVER['SERVER_SOFTWARE'] ) ) : '';
 
 		return $server_data;
 	}