Update istio/galley Docker tag to v1.5.8

This MR contains the following updates:

Package	Type	Update	Change
istio/galley	ironbank-docker	patch	1.5.7 -> 1.5.8
istio/galley		patch	1.5.7 -> 1.5.8
istio/galley	stage	patch	1.5.7 -> 1.5.8

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