Port Tails to Buster

This is a placeholder ticket so we can mark other tickets as blocking this one, even if they have a target version closer to us than 4.0.

Related issues

• Related to #16822 (closed)
• Blocked by #15857 (closed)
• Blocked by #15187 (closed)
• Blocked by #15747 (closed)
• Blocks #15010
• Blocks #16320 (closed)
• Blocks #6560
• Blocks #15786
• Blocks #15477
• Blocks #15450
• Blocks #16209
• Blocks #15946 (closed)

Original created by @intrigeri on 15944 (Redmine)