Allow omitting the top-level version property from the compose file

[suhaibmujahid]

The version property is obsolete, and we should support omitting it: https://github.com/compose-spec/compose-spec/blob/main/04-version-and-name.md

+ git clone --quiet https://github.com/suhaibmujahid/bugbug /code
+ cd /code
+ git -c 'advice.detachedHead=false' checkout 68a5381c22722572aed5d86d407469123c7b2244
HEAD is now at 68a5381c  -- 1
+ taskboot --target /code build-compose --write /images --build-arg 'CHECK_MODELS=0' --tag latest --tag latest
INFO:root:Target setup in /code
WARNING:taskboot.target:No target cloned
Traceback (most recent call last):
  File "/usr/local/bin/taskboot", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.10/site-packages/taskboot/cli.py", line 354, in main
    args.func(target, args)
  File "/usr/local/lib/python3.10/site-packages/taskboot/build.py", line 120, in build_compose
    assert version is not None, "Missing version in {}".format(composefile)
AssertionError: Missing version in /code/docker-compose.yml
[taskcluster 2026-05-19T19:47:14.571Z]                        Exit Code: 1