Tree view: Update expand/collapse component guidelines

[patternfly-jira-sync]

NOTE: Do NOT update the direction/behavior of the expand/collapse carets. This task is just for updating all images to show the new Brand UI icons.

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Jira Issue: PF-4376

[patternfly-jira-sync]

Closed via Jira sync - Jira issue PF-4376 was closed.