Improve performance of websocket_api _state_diff_event

[bdraco]

Proposed change

We can use last_updated_timestamp for the compare since its always calculated when the state is created and comparing floats is much faster than datetime objects

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[home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (websocket_api) you are listed as a code owner for? Thanks!

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