fix: use `json_encode` instead of `implode` in updateUes

Description

fix: use json_encode instead of implode in updateUes

Attribute values might be array, which implode did not handle and thus the log message caused errors (not affecting functionality, but spamming logs).

How to test

Deploy on proxy IdP instance and make sure at least one array attribute, which value is not serialized into string, is configured to be updated.

Author's checklist

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Reviewer's checklist

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Other information

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