Add missing Ubuntu os-release fixtures

[Mirochill]

Fixes #47

Summary:

• add missing Ubuntu os-release parse fixtures used by parse_os_release tests
• add missing Ubuntu LTS os-release sample files used by existing codename symlinks
• add matching distro expected fixtures for Ubuntu codename aliases

Validation:

• Not run locally
• git diff --check HEAD~1..HEAD
• Static fixture consistency checks: Ubuntu samples have parse and distro expected fixtures, JSON fixtures parse successfully, symlink targets exist, and generated parse/distro fixture data matches the os-release samples
• Remote CI: Azure aboutcode-org.container-inspector build 18871 passed; DCO passed