linux/update-mainline: fix oldest version check

    return f"sha256:{hash}"

def get_oldest_branch() -> Version:

    with open(VERSIONS_FILE) as f:

        return parse_version(sorted(json.load(f).keys())[0])

def get_oldest_branch(kernels) -> Version:

    return min(parse_version(v) for v in kernels.keys() if v != "testing")

def predates_oldest_branch(oldest: Version, to_compare: str) -> bool:

        if (parsed := parse_release(release)) is not None

    ]

    all_kernels = json.load(VERSIONS_FILE.open())

    oldest_branch = get_oldest_branch()

    oldest_branch = get_oldest_branch(all_kernels)

    for (branch, kernels) in groupby(parsed_releases, lambda kernel: kernel.branch):

        kernel = max(kernels, key=lambda kernel: kernel.parsed_version)