TPA-RFC-77: plan Merge of Tails' and Tor's Puppet servers

References:

• TPA-RFC-73: Tails infra merge roadmap - Puppet server
• TPA-RFC-77: Tails and TPA Puppet codebase merge

Steps:

• upgrade pauli to bookworm and the puppetmaster ... (#41819 - closed)
• come up with a plan (TPA-RFC-77)
  • detail costs (ref)
  • add a timeline (ref)
• split the tasks in bite-sized issues

This blocks:

• Migrate the Tails infra monitoring to Prometheus (#41946)