Removing deprecated Docker Version String from compose file

[tippexs]

PR Description

This PR removes the explicit version string (version: "3") from the docker-compose.yml.

Rationale

• Deprecated in Compose Specification
  The Docker Compose specification no longer requires or recommends using the version field. Starting with Compose v1.27.0 and the introduction of the Compose Specification, the version key has been deprecated.

• Improved Forward Compatibility
  By removing the version declaration, the file is parsed against the latest Compose specification by default. This ensures that new features and improvements introduced in Compose will be available without needing to bump the version manually.

• Reduces Maintenance Overhead
  Explicit version numbers can cause unnecessary churn and confusion, especially when developers assume they must be updated with new Docker releases. Omitting the field simplifies the file and avoids misleading future contributors.

• Broad Compatibility
  Modern Docker and Docker Compose versions handle files without version: seamlessly. Keeping it in place only creates the impression that specific constraints exist, which is no longer the case.

References

• Docker Docs: Compose file version and upgrading
• Compose Specification

Summary

This change aligns the Zammad docker-compose.yml with current Docker best practices, reduces confusion, and improves maintainability going forward.

[waja]

Duplicate of #480 and #415, which was reverted in #416

[tippexs]

Thanks wasn't checking the history. I would add a comment to the file, so it's clear WHY there is still a version tag? If thats good for you I will submit another commit within this PR?

[MrGeneration]

Maybe a simple comment above 'version x' that points to PR#416 might be a good idea to reduce the constant confusion from others.

[mgruner]

We should check this again. Maybe we can drop the version now when we do the change for Zammad 7.0.