[prod] Update teleport to v18 (major)

This MR contains the following updates:

Package	Type	Update	Change
teleport-cluster	prod	major	17.7.7 -> 18.2.6
teleport-kube-agent	prod	major	17.7.7 -> 18.2.6
teleport-plugin-event-handler	prod	major	17.7.7 -> 18.2.6
teleport-plugin-slack	prod	major	17.7.7 -> 18.2.6

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