Update istio/pilot Docker tag to v1.19.8

This MR contains the following updates:

Package	Type	Update	Change
istio/pilot		patch	1.19.7 -> 1.19.8
istio/pilot	ironbank-docker	patch	1.19.7-distroless -> 1.19.8-distroless
istio/pilot	stage	patch	1.19.7-distroless -> 1.19.8-distroless

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