Previously, to address issue containerd#11708, PR containerd#11793 changed containerd to always
invoke the shim binary to establish shim connections, rather than reusing the
sandbox shim. However, this change did not ensure that the Shutdown API was
called to stop the shim process.

Starting with containerd v2.0.0, the Shutdown API is only invoked for sandbox
containers (when container.SandboxID is empty). This approach works for
groupable shims, where multiple containers share a single socket address and
only require a single Shutdown call. However, for non-groupable shims, each
container requires its own Shutdown call during cleanup to avoid leaking shim
processes.

Additionally, PR containerd#11793 introduced a corner case during upgrades:
- T1: An old container-shim-runc-v2 (<=v1.7.X) is running for pod A.
- T2: containerd is upgraded to v2.X.Y.
- T3: A new container A-C1 is created in pod A using the new shim-runc-v2 binary.
- T4: bootstrap.json indicates version:3 protocol, but it is downgraded to version:2 in memory.
- T5: containerd is restarted.
- T6: containerd fails to connect to A-C1.
- T7: The A-C1 container is left in EXITED status in the CRI plugin.

To address this, ensure that loadShimTask downgrades to version:2 if necessary,
and always invoke the Shutdown API for each non-groupable shim during cleanup to
prevent resource leaks and handle upgrade scenarios correctly.

(Introduced by containerd#11793)

Signed-off-by: Wei Fu <[email protected]>