remove version from docker-compose.yml

[iamspido]

With docker version 25.05 the version 2.1 in docker-compose.yml will be obsolete.
docker-compose.yml: version is obsolete

Fix for #5797

[milkmaker]

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[JiiPee74]

This is only warning and removing it from config file may break older docker installations, so I would leave it for now.

[tomudding]

This is only warning and removing it from config file may break older docker installations, so I would leave it for now.

The version attribute has been optional since the new Compose specs released at the end of 2020, see https://github.com/docker/compose/releases/tag/1.27.0-rc1 and compose-spec/compose-spec@649d3b0.

Any recent version of Mailcow requires Compose v2 to be able to work, which has the above change since its inception. So if someone is still using a version of Compose that uses/only supports the v1 spec there are likely more serious issues with that system that require attention.