ROSAENG-61412 | fix: align y-stream upgrade tests with channel flow (#3353)

Add shared y-stream upgrade preparation helpers that compute and apply
the next-minor channel, then wait for a next-minor upgrade target to
appear before the tests proceed.

This aligns the e2e upgrade cases with the documented ROSA upgrade flow,
removes duplicated channel setup, and keeps dedicated upgrade lanes red
when no real upgrade path can be exercised.

Author: olucasfreitas