This change adds an information notice to the customizer when a block theme is active and the customizer is also available (for example when a plugin activates it), to encourage people to use the Site Editor for the best full site customization experience.
Props ironprogrammer, antonvlasenko, Clorith, audrasjb, psmits1567, tobifjellner, costdev, webcommsat.
Fixes#54939.
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