Upgrade Stackdriver Logging Agent addon image from 1.6.0 to 1.6.8 to fix segmentation fault bug.

[qingling128]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
Upgrade Stackdriver Logging Agent addon image from 1.6.0 to 1.6.8 to fix segmentation fault bug.

Which issue(s) this PR fixes:

Fixes segmentation fault issue with Protobuf library when log entries are deeply nested.

Special notes for your reviewer:
The new Stackdriver addon pipeline has been removed from this repo. This PR is to back port this fix to the legacy pipeline so that customers who have not yet been able to migrate still get the segmentation fault fixed.

Does this PR introduce a user-facing change?:

Fixes segmentation fault issue with Protobuf library when log entries are deeply nested.

[k8s-ci-robot]

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[jingax10]

/assign freehan

[jingax10]

/ok-to-test

[freehan]

/lgtm
/approve

[freehan]

/assign bowei@ for approval

[bowei]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bowei, freehan, qingling128

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[fejta-bot]

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