[EPIC] Upgrade to Liquidsoap 2.0 / 2.1

• If not running in Docker: Ubuntu 20+ or Debian equivalent
• Update Dockerfile to new Liquidsoap 2.1 release (no e.t.a. yet)
• Test and upgrade other dependencies in Dockerfile where applicable
• Update to new language specifics (references etc.)
• Max volume is now 1 instead of 100
• Optional arguments are not allowed anymore
• New JSON builder & parser
• input.http now includes HTTPS
• Metadata events on sources are now attached without returning another source, but a callback function. Edit: In Liquidsoap 2.1 the old implementation got re-introduced. Note: both behave differently
• audio_to_stereo is not needed anymore
• Update to new settings format
• Adapt default settings as per Liquidsoap 2 behavior. Especially in regards of frames and ALSA settings
• Simplified ReplayGain operator
• Get rid of warnings
• meta: Update Docker Compose configs in meta with current configs
• engine-core: Merge branch lqs2-migration to main
• engine: Merge branch lqs2-migration to main
• meta: Merge branch develop to main

Other related tasks

• Refactor clearing of queues (Liquidsoap 2) (#16 - closed)
• Latency issues for live audio when running Engi... (#6)