Filter identifier doesn't work for grouped taxonomy filter

Problem/Motivation

If the filter identifier for a taxonomy grouped filter is changed, the filter won't work.

Steps to reproduce

1. Create a view
2. Create a filter using taxonomy terms
3. Set this filter as grouped
4. Change the Filter Identifier

Proposed resolution

Include a condition on \Drupal\taxonomy\Plugin\views\filter\TaxonomyIndexTid::validateExposed to get the identifier when the filter is a group.