migrate gitlab-02 to new gnt-dal cluster

we're going to host more and more gitlab stuff in object storage (e.g. #41425 (closed)) and already have runners there. it makes sense to move gitlab-02 to the new gnt-dal cluster, which has faster disks and more powerful CPUs.

this should help us deal with the current overload in the gnt-fsn cluster as well (incident #41429 (closed)).

plan:

• communicate date of outage to all of tor
• add planned maintenance item on status.tpo
• lower TTL for subdomains that need it to 5mins -- gitlab-02 (aliases which don't need to change: gitlab, containers, gitaly), *pages
• prepare dal cluster for accepting instances from fsn, if needed (e.g. RAPI cert, RAPI passwords, firewall)
• zerofree on the backup partition (@anarcat)
• zerofree on the other partitions (not done because requires downtime, as it requires readonly partition)
• wipe free space on volume group
• on the day of the maintenance window
  • run steps 5 and 6 of prep
  • stop puppet on ganeti nodes on the dal cluster, finalize RAPI prep
  • stop the instance and start the transfer
  • change IP in instance after the move
  • test that gitlab and all other websites are replying properly, reconfigure grub-pc, test reboot of instance
  • switch DNS entries to point to new IP
  • schedule destruction of old instance
• after the move is finished
  • communicate to everyone that the move is over and that things are back to normal operation
  • remove password files and cert files created by the prep before migration
  • bring the TTL back to the default value of 1h for gitlab-02 and *pages