[WIP] Drop support for (archived) Compose-on-Kubernetes

[thaJeztah]

carries #3139
closes #3139

closes #2967
(https://docker.atlassian.net/browse/IL-257 for internal tracker)

- What I did
Removed support for Kubernetes orchestrator, as required compose-on-kubernetes backend is archived

- How I did it
Code removal

- How to verify it
isn't this what CI is for ? :P

- Description for the changelog
Removed support for deprecated Compose-on-Kubernetes

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Ok, at least some linting errors;

#17 66.46 cli/command/system/version.go:143:31: SA4017: HasKubernetes is a pure function but its return value is ignored (staticcheck)
#17 66.46 	if orchestrator.HasKubernetes() {
#17 66.46 	                             ^

And various (e2e) tests failing due to the kubernetes fields no longer being there;

=== FAIL: e2e/context TestContextImportTLS (0.05s)
    context_test.go:62: assertion failed: 
        --- expected
        +++ actual
        @@ -1,4 +1,4 @@
        -NAME        DESCRIPTION                               DOCKER ENDPOINT               KUBERNETES ENDPOINT                                 ORCHESTRATOR
        -default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock                                                       swarm
        -test                                                  unix:///var/run/docker.sock   https://kubernetes.docker.internal:6443 (default)   swarm
        +NAME        DESCRIPTION                               DOCKER ENDPOINT               ORCHESTRATOR
        +default *   Current DOCKER_HOST based configuration   unix:///var/run/docker.sock   swarm
        +test                                                  unix:///var/run/docker.sock   swarm
         
        
        
        You can run 'go test . -test.update-golden' to automatically update testdata/context-ls-tls.golden to the new expected value.'

[thaJeztah]

Some failures remaining (and probably docs and completion scripts to be updated as well), but getting closer

--- FAIL: TestCreateInvalids (0.00s)
    --- FAIL: TestCreateInvalids/orchestrator-all-no-endpoint (0.00s)
        create_test.go:120: assertion failed: expected an error, got nil
--- FAIL: TestExportImportWithFile (0.00s)
    export-import_test.go:23: assertion failed: error is not nil: context "test" does not exist
--- FAIL: TestExportImportPipe (0.00s)
    export-import_test.go:49: assertion failed: error is not nil: context "test" does not exist
FAIL

[thaJeztah]

need to squash the commits, as the first commit doesn't build (removes github.com/docker/cli/cli/context/kubernetes, but it's still used in some places in that commit)

[thaJeztah]

Did a rebase;
These still fail, so need fixing;

 === FAIL: cli/command/context TestCreateInvalids/orchestrator-all-no-endpoint (0.00s)
     create_test.go:120: assertion failed: expected an error, got nil
     --- FAIL: TestCreateInvalids/orchestrator-all-no-endpoint (0.00s)

 === FAIL: cli/command/context TestCreateInvalids (0.00s)

 === FAIL: cli/command/context TestExportImportWithFile (0.00s)
     export-import_test.go:23: assertion failed: error is not nil: context "test" does not exist

 === FAIL: cli/command/context TestExportImportPipe (0.00s)
     export-import_test.go:49: assertion failed: error is not nil: context "test" does not exist