upgrade gnt-fsn Ganeti cluster to bookworm

upgrade the gnt-fsn cluster to bookworm, which means those nodes

• fsn-node-01
• fsn-node-02
• fsn-node-03
• fsn-node-04
• fsn-node-05
• fsn-node-06
• fsn-node-07
• fsn-node-08

mind the Ganeti upgrade procedure.

possibly consider upgrading the (smaller?) gnt-dal cluster (#41253) first.