Deprecate RCTGetMultiplierForContentSizeCategory (#39643)

Summary:
Per #39617 (comment) , let's deprecate this method first.
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## Changelog:

[IOS] [DEPRECATED] - Deprecate RCTGetMultiplierForContentSizeCategory

Pull Request resolved: #39643

Test Plan: CI should pass

Reviewed By: dmytrorykun

Differential Revision: D49637172

Pulled By: cipolleschi

fbshipit-source-id: 20dc70c737ef51efd5a5aa38832fb47b5d49e641

packages/react-native/React/UIUtils/RCTUIUtils.h

@@ -25,7 +25,7 @@ extern __attribute__((visibility("default"))) RCTDimensions RCTGetDimensions(CGF
 
 // Get font size multiplier for font base size (Large) by content size category
 extern __attribute__((visibility("default"))) CGFloat RCTGetMultiplierForContentSizeCategory(
-    UIContentSizeCategory category);
+    UIContentSizeCategory category) __deprecated;
 
 #ifdef __cplusplus
 }