Add Index Upgrade feature

[DmitryKey]

Since luke allows opening index format of older versions (luke 7.1 opens 6.6.0 index), then it could offer Index Upgrade functionality via https://lucene.apache.org/solr/guide/7_1/indexupgrader-tool.html

[mocobeta]

I have never used this API so far, but I think it should be possible to add "Upgrade index" menu item to Tools menu.

Just leave my implementation image:

• Call IndexUpgrader.upgrade() method asynchronously
• Use PrintStreamInfoStream to output information to JTextArea

Also, It would be a good first issue for new contributors.

[mocobeta]

For contributors: Now Luke is a part of Apache Lucene. Please create an issue (if there is not exist) to Lucene Jira if you want to help with this.