Remove reference to `react-native upgrade` (#53265)

Summary:
This command doesn't exist anymore.

Remove reference to `react-native upgrade` as the command doesn't exist anymore.

## Changelog:

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[INTERNAL] [CHANGED] - Removed reference to removed `react native upgrade` in `Libraries/Renderer/README.md`

Pull Request resolved: https://github.com/facebook/react-native/pull/53265

Test Plan: N/A

Reviewed By: cipolleschi

Differential Revision: D80251577

Pulled By: cortinico

fbshipit-source-id: 9eab037af6769f0d1f78f65bc795d285dc3d8320