Merge branch '6237-amavis-permission' into 'develop'

Set amavis group for 50-user to make it readable in Debian 11

Closes #6237

See merge request ispconfig/ispconfig3!1530

install/lib/installer_base.lib.php

@@ -1659,6 +1659,7 @@ class installer_base {
 		$content = str_replace('{mysql_server_ip}', $conf['mysql']['ip'], $content);
 		wf($conf['amavis']['config_dir'].'/conf.d/50-user', $content);
 		chmod($conf['amavis']['config_dir'].'/conf.d/50-user', 0640);
+		chgrp($conf['amavis']['config_dir'].'/conf.d/50-user', 'amavis');
 
 		// TODO: chmod and chown on the config file