build: Update dependencies and build configuration

- Updated `flake.nix` file
- Updated `versionTool` URL to `https://gitlab.schukai.com/oss/utilities/version/-/archive/0.5.0/version-0.5.0.tar.gz`
- Removed commented out `versionTool` hash
- Modified `ldflags` value to include new `gitlab.schukai.com/oss/utilities/documentation-manager` URL
- Removed `vendorSha256` line
- Added `versionTool.defaultPackage.${system}` to `buildInputs` list