Update system requirements for lizard failover

Originally created by @intrigeri on #10243 (Redmine)

Our needs have changed quite a bit since we’ve specified these requirements 2 years ago (tails/sysadmin#6186).

Blueprint: https://tails.boum.org/blueprint/lizard_failover/#system-specs

Parent Task: tails/sysadmin#6185

Related issues

• Related to tails/sysadmin#6186
• Related to tails/tails#9400
• Blocks tails/sysadmin#10244