Fix LDAP group mirroring

[Paktosan]

This fixes an error likely introduced in 04c18cd which prevents configuring a working LDAP setup with this chart.
Currently the chart forces mirrorGroupsExcept to be a string which results in Netbox throwing the following error when trying to log in:

<class 'django.core.exceptions.ImproperlyConfigured'>

AUTH_LDAP_MIRROR_GROUPS_EXCEPT must be a collection of group names

With the proposed change, the correct type is set in the schema and a sensible default in the values.yaml is added.