How to preprocess group title render in views?
If you want to change the template for the title of the group, why don't you override the template file views-view-grouping.tpl.php for Drupal 7, or views-view-grouping.html.twig for Drupal 8.

By standard Drupal templates names, overrides are also working with this one.

views-view-grouping--MACHINEVIEWNAME.tpl.php for a specific view

views-view-grouping--MACHINEVIEWNAME--MACHINEDISPLAYNAME.tpl.php for a specific display of a specific view.

Hope this can help you.