Follow-up to [52165] where the `version_compare()` fails for 11.8.x versions. This commit changes the version comparison to < 11.9 for deactivating the Gutenberg plugin.
The `_upgrade_580_force_deactivate_incompatible_plugins()` function is no longer needed in 5.9. It's redundant and unnecessary as `_upgrade_590_force_deactivate_incompatible_plugins()` deactivates those versions as well.
Removing `_upgrade_580_force_deactivate_incompatible_plugins()` and moving the deactivation logic back into the `_deactivate_gutenberg_when_incompatible_with_wp()`, thus removing the new private function `_deactivate_gutenberg_when_incompatible_with_wp()` introduced in [52165].
Follow-up [51180], [51266], [52165].
Props hellofromTonya, tobiasbg, clorith, sergeybiryukov, costdev.
Fixes#54405.
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