Remove findNodeHandle and measure*() New Architecture Migration Steps

[yungsters]

Updates the New Architecture migration guide in the following ways:

• Removed the findNodeHandle and measure*() migration steps.
  • Backward compatibility landed in facebook/react-native@455ca8c.
• Rewrote the introduction for this section of the migration steps.
  • Added more details on our plans to eventually deprecate UIManager, but to continue supporting it (and the development is actively ongoing).
  • Omit references to the deprecation of findNodeHandle.
  • In lieu of removing the code snippets from the obsolete measure*() section, I've included a code snippet demonstrating usage of the native component instance methods.

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[yungsters]

I think we should wait until 0.71 is released before merging this.

Also, I just remembered that the code snippet should be using viewRef.current.measure(…), not viewRef.measure(…).

[cipolleschi]

It looks very good to me and it is refreshing to see so much "code" removed! :D

I think we should wait until 0.71 is released before merging this.

It is already like this. When we merge this into main, the documentation is published as next version. When RN 0.71 is released, the next is automatically promoted to 0.71 and the new next will refer to 0.72. 😉

[yungsters]

I just spotted a typo, "to continuing". Will fix (and subsequently merge) when I'm back from my vacation! Thanks.