Add metabox flag for RTC-compatible metaboxes #29008

Job	Run time
Core running from src / MacOS	1m 23s
Core running from build / MacOS	1m 22s
Core running from src / Windows	1m 20s
Core running from build / Linux	1m 38s
Core running from src / Linux	1m 8s
Core running from build / Windows	1m 22s
Slack Notifications	-1s
Failed workflow tasks	1s
	8m 13s

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