fix #6516: ui: backup job: limit the job ID length

The API only allows backup job IDs of up to 50 characters, so mark
longer values as invalid in the edit window already, instead of failing
on submit.

Signed-off-by: Michal Fox <me@dualfroz.com>

www/manager6/panel/BackupAdvancedOptions.js

@@ -122,6 +122,7 @@ Ext.define('PVE.panel.BackupAdvancedOptions', {
             startColumn: {
                 xtype: 'pmxDisplayEditField',
                 vtype: 'ConfigId',
+                maxLength: 50,
                 fieldLabel: gettext('Job ID'),
                 emptyText: gettext('Autogenerate'),
                 name: 'id',