docs: add changelog for PR 12016 (license key enhancement)

changes/ee/feat-12016.en.md

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+Enhanced license key management.
+
+EMQX can now load the license key from a specified file. This is enabled by setting the `license.key` configuration to a file path, which should be prefixed with `"file://"`.
+Also added the ability to revert to the default trial license by setting `license.key = default`. This option simplifies the process of returning to the trial license if needed.