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