Make Groups the default value for `group_attribute` in SAML configuration

Proposal

For SAML Group sync to work, the customer is required to do the following:

1. Configure SAML
2. Configure SAML Group link
3. Make sure group_attribute is set to Groups

This is error prone as groups_attribute can be misplaced. See !108663 (merged)

A customer suggests that we use Groups as group_attribute default value.