Update api minimum Go version to 1.21

[dmcgowan]

A higher go build version is not required for the API and this will help some downstream projects test the change which are currently blocked on 1.22 upgrade.

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[thaJeztah]

So, it does not immediately refuse it now, but it looks like API 1.8 is not backward compatible from a Go perspective. It looks like some parts were removed that the 1.7 code expects (perhaps moved elsewhere?)

24.59 # github.com/containerd/containerd/services/introspection
24.59 vendor/github.com/containerd/containerd/services/introspection/local.go:88:35: cannot convert &Local{} (value of type *Local) to type "github.com/containerd/containerd/api/services/introspection/v1".IntrospectionClient: *Local does not implement "github.com/containerd/containerd/api/services/introspection/v1".IntrospectionClient (missing method PluginInfo)
24.59 vendor/github.com/containerd/containerd/services/introspection/service.go:57:12: cannot use localClient (variable of type *Local) as "github.com/containerd/containerd/api/services/introspection/v1".IntrospectionClient value in struct literal: *Local does not implement "github.com/containerd/containerd/api/services/introspection/v1".IntrospectionClient (missing method PluginInfo)
24.85 # github.com/containerd/containerd/sandbox
24.85 vendor/github.com/containerd/containerd/sandbox/bridge.go:35:10: cannot use &grpcBridge{…} (value of type *grpcBridge) as "github.com/containerd/containerd/api/runtime/sandbox/v1".TTRPCSandboxService value in return statement: *grpcBridge does not implement "github.com/containerd/containerd/api/runtime/sandbox/v1".TTRPCSandboxService (missing method SandboxMetrics)
24.85 vendor/github.com/containerd/containerd/sandbox/bridge.go:45:33: cannot use (*grpcBridge)(nil) (value of type *grpcBridge) as "github.com/containerd/containerd/api/runtime/sandbox/v1".TTRPCSandboxService value in variable declaration: *grpcBridge does not implement "github.com/containerd/containerd/api/runtime/sandbox/v1".TTRPCSandboxService (missing method SandboxMetrics)
47.61 # github.com/containerd/containerd/runtime/v2/shim
47.61 vendor/github.com/containerd/containerd/runtime/v2/shim/publisher.go:39:12: undefined: v1.Envelope
47.61 vendor/github.com/containerd/containerd/runtime/v2/shim/publisher.go:118:11: undefined: v1.Envelope
59.83 + rm -f /go/src/github.com/docker/docker/go.mod

[thaJeztah]

So for the module to be usable for 1.x, (1.7), it looks like some changes are needed in the 1.7 code 🤔

[dmcgowan]

We can bring in those changes to make it compatible, testing with #10278

This PR doesn't depend on that though and can be considered on its own.

[thaJeztah]

LGTM