Use Attribute::addClass() instead of CSS class concatenation in template_preprocess_views_view_table()

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

Points in #24 still need to be addressed.

Hoping this will help, here's how I can reproduce this error:

1. Create a View with Table display and add some fields
2. Exclude some fields from display or set some fields to the column of another field, save
3. Change the Responsive (High/Medium/Low) of some these columns, save
4. Get Error Notice: Indirect modification of overloaded element of Drupal\Core\Template\Attribute has no effect in template_preprocess_views_view_table() (line 569 of /var/www/html/web/core/modules/views/views.theme.inc)

Note that:

• You cannot remove the error until you remember or by chance set the previous columns' responsive class
• A workaround to stop the error is to change the view display and set it back to Table to reset those settings (fields' settings will stay)

Is there anything left here that's not covered by 🐛 Indirect modification of overloaded element with Views responsive table Fixed ?

Patch for 11.x.
Pointer in #24 still needs to be addressed.