Return an array for set cookie header as a default

[ztefanie]

Describe the Bug

When updating from 8.5 to 8.6.0 no longer multiple set cookie headers are returned as an array, but only the first one.
With 8.6.7 it is possible to get all set cookie headers as an array when using the groupSetCookieHeaders flag. I just confirmed the output of 8.5 and latest with the flag is the same.

Purpose of this issue:

• 8.7 and 8.8. should return an array per default, without the need of the flag.
• next 8.6.11 / next minor should also return an array per default

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

Both potentially breaking changes should be documented.

Support issues:
https://jira.camunda.com/browse/SUPPORT-25027
https://jira.camunda.com/browse/SUPPORT-26056