SolrIndex client clean all collections when droping a graph

Actually droping a graph in Gremlin console empties all the collections of Solr of the others graphs.

- Storage Backend: inmemory
- Mixed Index Backend: solr
- Link to discussed bug: [lfai](https://lists.lfaidata.foundation/g/janusgraph-users/topic/80377377?p=Created,,,20,1,0,0::recentpostdate/sticky,,,20,2,0,80377377https://lists.lfaidata.foundation/g/janusgraph-users/topic/80377377?p=Created,,,20,1,0,0::recentpostdate/sticky,,,20,2,0,80377377)
- Expected Behavior: Droping a graph cleans/removes only its corresponding index leaving the others unchanged
- Current Behavior: Droping a graph cleans all solr/zookeeper collections
- Steps to Reproduce: Create a graph with a solr index with data. Create a second graph, drop it and check that the solr index of the first graph is now empty

Responsible code maybe the method clearStorage from [SolrIndex.java line 971](https://github.com/JanusGraph/janusgraph/blob/9aff93847866c0df725592c1975e56ccd8e1020a/janusgraph-solr/src/main/java/org/janusgraph/diskstorage/solr/SolrIndex.java#L971)

Start in janusgraph-solr/src/main/java/org/janusgraph/diskstorage/solr/SolrIndex.java at the clearStorage method around line 971. Reproduce the two-graph scenario described in the issue, then trace how dropping one graph selects Solr and ZooKeeper collections. Done means dropping one graph removes only its corresponding index while the other graph's collection and data remain unchanged.

Written by the indexing model from the issue text.