Update tensorflow/serving Docker tag to v2.6.0

This MR contains the following updates:

Package	Type	Update	Change
tensorflow/serving	ironbank-docker	minor	2.5.1 -> 2.6.0
tensorflow/serving		minor	2.5.1 -> 2.6.0
tensorflow/serving	stage	minor	2.5.1 -> 2.6.0

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