SYSADMIN: Upgrade BTCPay to Debian 11

[nochiel]

(Debian 9 moved out of long-term support at the end of June, 2022.)

• [x] Make backup snapshot.
• [x] Upgrade boot configuration to GRUB 2.
• [x] Upgrade to Debian Stable.
• [x] Restart BTCPay server docker container
  • [x] Verify linode is reachable.
  • [x] Update docker
• [x] Update BTCPay
• [x] Tests
  • [x] Test that invoices are generated on mainnet.
  • [x] Test that the donations are registered correctly.
  • [x] Test for regressions. (Get feedback from @ChristopherA and @shannona)
  • [x] Fix broken CSS. (Thanks @shannona!)
  • [x] Add a few payment amount options to make the payment flow legible.