Upgrade xwiki.org wiki farm to 11.10.x

See https://www.xwiki.org/xwiki/bin/view/XWiki/Migrations/ for detail on some commands and what was done last time.

Steps to discuss for next time:

• solr.xwiki.org
  • clone VM
  • upgrade to Debian Buster
  • upgrade to Solr 8.1.1
  • upgrade xwiki conf to 11.10.x

• xwikiorg-node1
  • clone VM
  • upgrade to Debian Buster
  • move to Tomcat 9 Debian package
  • upgrade XWiki WAR to 11.10.x
  • disable cluster
  • change solr host

• migration dry run
  • clone xwikiorg db VM
  • upgrade to Debian Buster (which include upgrade MySQL server)
  • change node1-new target DB
  • start node1-new, migrate/upgrade and make sure everything is OK
  • XAR export the hand made modifications/fixes to import them when doing for the real upgrade
  • attachment store: either use attachment database store or setup some network shared folder

• actual migration
  • stop node1-new
  • set node1/node2 in readonly and restart
  • copy just the database data or redo clone VM + Debian Upgrade ?
  • backup/restore node1 permanent directory to node1-new
  • do the real upgrade
  • import the XAR containing the fixes/improvements done during the dry run

• node2
  • create a new node based on node2-new

• cluster
  • change cluster IPs on both nodes
  • reconfigure the load balancer to point on the two new nodes