Update docs to recommend against specifying the version in docker-compose.yml

[rseymour]

The original docker compose version was apparently just taken as a suggestion and never validated, and was quietly decommissioned in a recent docker release. The docs still suggest using it to prevent confusion. To prevent confusion (for real) perhaps the docs should note that version never meant anything to the parser logic anyyway and so it has been removed.

          Could the docs be updated?

Originally posted by @rseymour in #11628 (comment)

[rseymour]

opened a docs issue: docker/docs#19684