Backport: Block theme previews #23245

Job	Run time
5.6 on ubuntu-latest	7m 41s
5.6 multisite on ubuntu-latest	8m 15s
7.0 on ubuntu-latest	8m 6s
7.0 multisite on ubuntu-latest	7m 2s
7.1 on ubuntu-latest	7m 12s
7.1 multisite on ubuntu-latest	9m 13s
7.2 on ubuntu-latest	6m 47s
7.2 multisite on ubuntu-latest	8m 44s
7.3 on ubuntu-latest	10m 16s
7.3 multisite on ubuntu-latest	11m 54s
7.4 on ubuntu-latest	7m 10s
7.4 multisite on ubuntu-latest	11m 7s
8.0 on ubuntu-latest	8m 31s
8.0 multisite on ubuntu-latest	7m 38s
8.1 on ubuntu-latest	7m 38s
8.1 multisite on ubuntu-latest	7m 43s
8.2 on ubuntu-latest	6m 29s
8.2 multisite on ubuntu-latest	7m 28s
5.6 slow tests on ubuntu-latest	9m 32s
5.6 multisite slow tests on ubuntu-latest	10m 28s
7.4 with memcached on ubuntu-latest	8m 8s
7.4 multisite with memcached on ubuntu-latest	9m 0s
Slack Notifications	0s
Failed workflow tasks	0s
	3h 6m 2s

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