Debian Bullseye support

[Kixunil]

We want to support Bullseye ASAP.

• [x] Try to rebuild all packages for bullseye (note: cargo and npm upgrades)
• [x] Test all packages
• [ ] Fix all compatibility issues
• [ ] Check determinism
• [x] Create a new repository for bullseye
• [x] Buy more space to host the repository
• [x] Publish repository as experimental
• [ ] Get reckless volunteers to test it
• [ ] Test on main testing machine
• [x] Test upgrade scenario from beta
• [ ] Release into beta
• [ ] Announce the change and deprecate Buster

Expected Bullseye release date 14 August 2021

[Kixunil]

Build result:

• dh-systemd was moved into debhelper - had to be replaced in debcrafter
• electrs needed build-dep version bump libstdc++-8-dev -> libstdc++-10-dev
• ThunderHub doesn't build - complains about something being outdated, the suggested command doesn't fix it. Let's try to update it first.
• remir doesn't build - didn't inspect since it's unimportant package

Test result:

• ✔️ multi-package-btcpayserver-regtest

Other packages not tested yet, presumably all deps of btcpayserver && lnd work fine

[Kixunil]

Tested upgrade from buster-experimental: everything went well except LND was probably restarted before bitcoin-rpc-proxy.

https://github.com/Kixunil/debcrafter/issues/54