Docs: Update generating test coverage reports (#29255)
Docs: Update generating test coverage reports (#29255)

Old docs said to use maven. That doesn't work. We can't generate the
reports right now.
GlenRSmith/elasticsearch,gfyoung/elasticsearch,rajanm/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,rajanm/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,rajanm/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,rajanm/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,gfyoung/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,rajanm/elasticsearch,rajanm/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch