SolrBackup should call DELETEBACKUP with purgeUnused=true

[killdash9]

When keeping only the most recent n backups, orphaned files can accumulate in the backup repository. Calling

/solr/admin/collections?action=DELETEBACKUP&name=myBackupName&location=/path/to/my/shared/drive&purgeUnused=true will clean those up. See https://solr.apache.org/guide/solr/latest/deployment-guide/collection-management.html for documentation on this api call.

The SolrBackup CRD should call this api as part of managing recurring backups.

This was part of the original plan in #359 but it was abandoned due to a bug with that api which has since been fixed. Details are in #359's comment threads.