Add a note about deprecating /register with a user property. (#15703)

Application services providing a "user" property (instead of "username") for
the /register endpoint was never specified. Deprecate this very old
fallback.

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diff --git a/changelog.d/15703.removal b/changelog.d/15703.removal
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+Deprecate calling the `/register` endpoint with an unspecced `user` property for application services.