Fixed incorrect opacity behaviour for <Text> component on iOS (#24435)

Summary:
This PR fixes #24229.
Seems currently `opacity` props for Text is being applied twice (one for text color and one for the whole view). This PR disables applying the prop to the text.

[CATEGORY] [TYPE] - Fixed double applying opacity prop for Text
Pull Request resolved: https://github.com/facebook/react-native/pull/24435

Differential Revision: D14932795

Pulled By: cpojer

fbshipit-source-id: f9280fc75f788424cb5f1e42d2e79efdb354d645

Libraries/Text/Text/RCTTextShadowView.m

@@ -46,6 +46,7 @@
   // the RCTTextView backgroundColor to be used, without affecting nested Text
   // components.
   self.textAttributes.backgroundColor = nil;
+  self.textAttributes.opacity = NAN;
 }
 
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