Update 'cannot calculate shadow efficiently' log message (#39700)

Summary:
While working in an app I kept getting these `View X of type Y has a shadow set but cannot calculate shadow efficiently. Consider setting a background color to fix this` warnings even though I had added a background color to that view. Upon inspecting RCTView.m I notice that what is actually required to fix this is a solid background

To make this a bit clearer to developers I believe we should update this log message to explicitly say "solid background" instead of "background"

## Changelog:

[IOS] [CHANGED] - Update 'cannot calculate shadow efficiently' log message to explicitly say solid background

Pull Request resolved: #39700

Test Plan: N / A

Reviewed By: christophpurrer

Differential Revision: D51584574

Pulled By: javache

fbshipit-source-id: b1741f7002ebb876e4a50959bef7f39df76a5c3c

packages/react-native/React/Views/RCTView.m

@@ -883,7 +883,7 @@ static void RCTUpdateShadowPathForView(RCTView *view)
 
       RCTLogAdvice(
           @"View #%@ of type %@ has a shadow set but cannot calculate "
-           "shadow efficiently. Consider setting a background color to "
+           "shadow efficiently. Consider setting a solid background color to "
            "fix this, or apply the shadow to a more specific component.",
           view.reactTag,
           [view class]);