fix(rest-connector): Return always an array for set cookie headers

[ztefanie]

Description

Restore the default behavior of 8.5. Set cookie headers a always merged into an array. The flag for grouping is removed. This also means, if someone sets the group flag to false, this will be ignored after an update, but I think that is ok, as this is not really a helpful feature and I guess noone will do this.

This will be documented here

I did not document the "breaking change" in the last two lines of the table, because i consider this a bug fix and not a breaking change. Let me know if you disagree, then I will add it to the docs. (We could adjust the behavior so that if there is only one Set-Cookie header, it returns a simple string instead of an array. However, this approach is not ideal. For example, if an endpoint sometimes returns one cookie (e.g., when login fails) and sometimes two (e.g., when login succeeds), it would make handling cookies in FEEL more complex.)

Behavior	Expected result
Users are on 8.5 and update to 8.6.11	nothing should break
Users are on 8.5 and update to 8.6.7	will break but should be fixed with manually editing the template
Users are on 8.6.7 and did edit their template and update to 8.6.11	should work fine
Users are on 8.6.7 and did not edit their template and update to 8.6.11	breaking change: if using a set cookie header in a feel result, this will fail, as now an array is returned
Users are on 8.6.7 or higher and update to 8.7	breaking change: if using a set cookie header in a feel result, this will fail, as now an array is returned

Related issues

closes #4201

Checklist

• PR has a milestone or the no milestone label.