Clean up active configuration from alpha/beta versions

I've tested the update hook well, while updating config from our internal project.

@fago I want to do this, but.... since we have been "ping-ponging " several times already about the code change in EntityCeDisplay, I didn't want to commit this without you seeing that particular change.

With this MR, the "hidden: null" value is really truly gone, so it cannot cause errors anymore. (It can only cause errors as long as you update this code without having run the update hook yet.)

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Just some random details for documentation:

Behavior that has confused me for a long time already / what I learned:

• After the update hook runs, active configuration holds no "region" or "hidden" values anymore,
• BUT... a "drush config-export" will still write those values into new files in an empty config/sync directory! (Until you change a 'real' value in the config entity.) Reason:
  • It uses some temporary file storage internally, which is not updated until you actually change a value
  • Before exporting, the active configuration is compared with the temporary storage
  • Since (because of the config schema) no "real" differences are found... the temporary storage (which still contains "hidden: null" etc) is used for exporting!
• There is no good accessible method to access/clear this temporary storage, so I'm leaving it alone.
• If those files (containing "region" and "hidden: null" are re-imported, these values do not end up in active storage; they are filtered out (because of the config schema). So there's no risk of bugs there.

I've tested the update hook well, while updating config from our internal project.

I've now also tested several integration test runs (i.e. browser tests etc) with this changed code in place.

Automatically closed - issue fixed for 2 weeks with no activity.