Fix(spec): Remove obsolete version from docker-compose.yml

[scale03]

What is the purpose of this change?

This PR removes the obsolete version attribute from the top of the spec/docker-compose.yml file. This attribute is no longer required by modern versions of Docker Compose and was producing a warning message when running docker compose up.

Related Issues

• This change is related to issue #3392.

Important Note for Reviewers

While trying to test this change, I discovered that the developer environment build is currently failing due to an expired GPG key for the Puppet 6 repository.

This PR does not fix that issue, but I have opened a separate bug report for it here: [#3396]

The CI build for this PR will likely fail because of that unrelated error.

[CLAassistant]

All committers have signed the CLA.