Add warning about 3.0 #4114

Job	Run time
Flags (pinned) (3.13, ubuntu-22.04)	4m 0s
Flags (pinned) (3.9, ubuntu-22.04)	1m 45s
Flags (pinned) (3.7, ubuntu-22.04)	1m 34s
Flags (pinned) (3.12, ubuntu-22.04)	3m 31s
Flags (pinned) (3.8, ubuntu-22.04)	2m 13s
All pinned Flags tests passed	3s
	13m 6s

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