Renovate to use horizon-flake-parts 0.10.0

Updates to use horizon-flake-parts 0.10.0 which provides integrated modules for CI, formatting, and systems configuration.

Changes

• Add horizon-flake-parts 0.10.0 input
• Remove old inputs: gitlab-ci, horizon-supported-systems, treefmt-nix
• Remove old config files: nix/ci.nix, nix/systems.nix, nix/treefmt.nix
• Add nix/configuration.nix with horizon module configuration
• Keep .gitlab-ci.yml as required GitLab CI entry point
• Update all flake inputs to latest versions