[release/1.6] go.mod: Bump hcsshim to v0.9.6

[kiashok]

This bumps shim tag to include a fix from
microsoft/hcsshim @ 5d5b21088e7fc80eeea5172e573c8fa68f3de046

Change log for hcsshim v0.9.6 can be seen here.

[k8s-ci-robot]

Hi @kiashok. Thanks for your PR.

I'm waiting for a containerd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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