Fix flake.nix, add missing branches, update all inputs
- Remove dead code (duplicate outputs expression) and unused import-tree/nix/
- Fix horizon-core-ghc912x ref (was pointing to 9.10.x)
- Add horizon-core-ghc914x (lts/ghc-9.14.x)
- Add horizon-devtools-ghc96x (lts/ghc-9.6.x)
- Fix horizon-devtools entries missing .md extension
- Remove unused horizon-core and horizon-platform default branch inputs
- Use system-aware package evaluation instead of hardcoded x86_64-linux
- Update all flake inputs to latest