Merge branch '6311-add-warning-to-update-script-when-incorrect-php-version-is-used-as-default' into 'develop'

Resolve "Add warning to update script when incorrect PHP version is used as default"

Closes #6311

See merge request ispconfig/ispconfig3!1589

Author: Marius Burkard

install/lib/installer_base.lib.php

@@ -249,6 +249,7 @@ public function check_prerequisites() {
 		$msg = '';
 
 		if(version_compare(phpversion(), '5.4', '<')) $msg .= "PHP Version 5.4 or newer is required. The currently used PHP version is ".phpversion().".\n";
+		if(version_compare(phpversion(), '8.0', '>=')) $msg .= "PHP Version 8 is not supported yet. Change PHP version back to the default version of the OS. The currently used PHP version is ".phpversion().".\n";
 		if(!function_exists('curl_init')) $msg .= "PHP Curl Module is missing.\n";
 		if(!function_exists('mysqli_connect')) $msg .= "PHP MySQLi Module is nmissing.\n";
 		if(!function_exists('mb_detect_encoding')) $msg .= "PHP Multibyte Module (MB) is missing.\n";