sys: notify client-server API version mismatch

[maksimtech]

Description
This PR adds an explicit notification log when the Docker CLI client session API version is downgraded to match the daemon (server) version.

Currently, when the client supports a higher API version than the daemon, it silently downgrades the session API version to maintain compatibility. While this fallback mechanism works flawlessly, providing a clear note in the docker version output helps developers and administrators immediately notice that a protocol downgrade has occurred.

Changes
Modified cli/command/system/version.go.

Added a conditional check inside runVersion after the ServerVersion API call to compare the client's supported API version with the negotiated server API version.

Prints a clear message to dockerCLI.Err() if a mismatch/downgrade is detected:
Note: API version session negotiated to (Client supports ).

How to test
Build the binary locally: make binary

Run the compiled docker binary against a Docker Daemon that has a lower API version than the client:

Bash
./build/docker version
Verify that the warning note appears dynamically above the standard prettyPrintVersion output.

Checklist
[x] I have signed my commits with git commit -s (DCO checked).

[x] The code builds successfully locally.

[thaJeztah]

What problem are you trying to solve? The output already indicates if the client downgraded? e.g. when running a current version of the CLI against an old (23.x) daemon;

docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock docker:cli version
Client:
 Version:           29.6.1
 API version:       1.42 (downgraded from 1.55)
 Go version:        go1.26.4
 Git commit:        8900f1d
 Built:             Fri Jun 26 11:39:39 2026
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          23.0.4
  API version:      1.42 (minimum version 1.12)
...

[maksimtech]

Thanks for the review and for pointing that out!

My main goal was to expose this event to stderr (dockerCLI.Err()). While the pretty-printed stdout template works perfectly for human operators looking at the terminal, automated scripts, CI/CD runners, or wrappers driving the CLI programmatically often ignore or don't parse the stdout template structure.

Having a standardized stream notification on stderr allows automation tools to immediately catch that an API version session mismatch/downgrade occurred.

That being said, if you feel this introduces unnecessary noise or redundancy, I completely understand and I'm happy to close the PR!