Preprocess views view to concat two dates and add variable
hook_preprocess_views_view() is used to alter the variables that are passed to a views-view[...].html.twig template file which is for the entire View, not just for the fields. It might make more sense for you to use hook_preprocess_views_view_fields() or hook_preprocess_views_view_field() since you are trying to alter the variables that are used in a views-view-field[...].html.twig template file (specific to the individual field, not the whole view).

See:

https://api.drupal.org/api/views/theme%21theme.inc/function/template_preprocess_views_view_fields/7.x-3.x

https://api.drupal.org/api/views/theme%21theme.inc/function/template_preprocess_views_view_field/7.x-3.x