Add metabox flag for RTC-compatible metaboxes #34885

Job	Run time
Determine Matrix	4s
Single Site Default / Test current	13m 51s
Single Site Memcached / Test current	13m 57s
Single Site Memcached / Test before	13m 50s
Single Site Default / Test before	14m 7s
Multisite Memcached / Test current	13m 16s
Multisite Memcached / Test before	14m 11s
Multisite Default / Test current	13m 16s
Multisite Default / Test before	27m 21s
Compare / Multisite Default	17s
Compare / Single Site Memcached	10s
Compare / Single Site Default	14s
Compare / Multisite Memcached	11s
Slack Notifications	0s
Failed workflow tasks	0s
	2h 4m 45s

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