This commit changes the solr health indicator to request the status of
all available cores, rather than invoking a generic ping on the admin
handler.

Recent versions of Solr now have a ping handler per core and the general
admin handler is gone, leading to incorrect health status. For backward
compatible reason the `solrStatus` attribute is kept to `OK` if all cores
are running properly (that's actually `0` behind the scenes).

Closes gh-8463