breaking(transport): Make HTTP2Transport the default #8625

Job	Run time
Common (pinned) (3.13, ubuntu-22.04)	5m 30s
Common (pinned) (3.8, ubuntu-22.04)	5m 51s
Common (pinned) (3.9, ubuntu-22.04)	7m 41s
Common (pinned) (3.10, ubuntu-22.04)	5m 54s
Common (pinned) (3.7, ubuntu-22.04)	3m 48s
Common (pinned) (3.11, ubuntu-22.04)	5m 59s
Common (pinned) (3.12, ubuntu-22.04)	5m 50s
All pinned Common tests passed	4s
	40m 37s

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