Avoid a fatal error due to `WP_Theme_JSON_Schema` and potentially other classes and/or functions redeclarations when updating to WordPress 5.9 with an incompatible version of the Gutenberg plugin.
This commit uses the same strategy from 5.8. Moves the plugin deactivation code (introduced in [51266]) to a private function for reuse in 5.8, 5.9, and future major releases.
Follow-up to [51180], [51266].
Props hellofromTonya, johnbillion, jorbin.
See #54405.
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