Update tensorflow/serving Docker tag to v2.6.1

This MR contains the following updates:

Package	Type	Update	Change
tensorflow/serving	ironbank-docker	patch	2.6.0 -> 2.6.1
tensorflow/serving		patch	2.6.0 -> 2.6.1
tensorflow/serving	stage	patch	2.6.0 -> 2.6.1

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