Deprecate non-nullable `target` field on Todo type

What does this MR do and why?

Under certain circumstances like the need for SAML-reauthentication, the target on a Todo can be null.

Unfortunately the target field is not nullable, so we created a new one target_entity which is nullable.

I still don't understand why non-nullable -> nullable is a breaking change on a field because it relaxes a constraint, but here we are.

#484987

Changelog: changed

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