Removed obsolete version element from compose file.

[pbratkowski]

As per https://docs.docker.com/reference/compose-file/version-and-name/#version-top-level-element-obsolete:

The top-level version property is defined by the Compose Specification for backward compatibility. It is only informative and you'll receive a warning message that it is obsolete if used.

Now that the README refers to Compose V2, I believe it makes sense to remove the reference to the obsolete value, and suppress the warning.

[bmispelon]

Good call, thanks for the PR! 🎸

Would you mind opening another one for https://github.com/django/code.djangoproject.com/blob/main/docker-compose.yml too?

Thank!

[pbratkowski]

Sure thing, done :)