Current situation

• the pipeline can currently (in theory) build all versions ever released (even though not all git checkouts are locked to revision, so build results will change over time T340226)
• github actions CI (on which we rely for building the artifacts we actually release) can only build one version at a time (there are workarounds to trigger builds for other versions too, they are not obvious nor intuitive though)

Goal

• make the pipeline build all versions we currently support at the same time, that is (following the "channel" naming from T340228)
  • stable
  • oldstable / lts
  • next (prepared, will be part of T343692)
• remove the versions folder
• tag the pipeline main branch to visualize which pipeline revision can build which artifact versions - should be only done for actual releases, TODO: put this into some matching ticket
• make CI jobs run for all three versions
• [exploration] prepare to build "cutting edge" as the "next" version T343692
• tag built docker artifacts with version strings
• name built tar balls with version strings during upload
• create three "example" directories to visualize how to deploy those three versions T340234
• composer cache seems to be always empty - followup in T346882
• make sarif reports work again
• fix remaining failing tests
• upload to ghcr only when test succeeded
• combine as many artifacts
• review on which level the channels should be handled
• fix missing build metadata in artifacts
• review parent task docs and fix to reflect current strategy
• review whole repo docs for changes introduced with this PR
• do we need an ADR for this change?
• fix: docker save saves all image tags
• fix failing mw version test