fix: setting headerBackTitleVisible to false not working on iOS (#11937)

**Motivation**

According to [the
docs](https://reactnavigation.org/docs/7.x/native-stack-navigator#headerbacktitlevisible),
setting `headerBackTitleVisible` to `false` should hide the back title
on iOS, while this is not the case with `react-native-screens` v3.29+.

**Cause**

With the current code, `backTitle={headerBackTitleVisible ?
headerBackTitle : ' '}`, the back title is set to a blank string if
`headerBackTitleVisible` is false. However, in newer versions of
`react-native-screens`, [the `backTitle` will be considered blank if it
only contains white
spaces](https://github.com/software-mansion/react-native-screens/blob/d54a19a/ios/RNSScreenStackHeaderConfig.mm#L506),
making it fallback to the default title - which makes the back title not
hidden at all.

**Fix**

Assign [the `backTitleVisible` property of `RNSScreenStackHeaderConfig`
native
component](https://github.com/software-mansion/react-native-screens/blob/d54a19a9789b799566da16a89e2cd8d8f1ad0ba7/ios/RNSScreenStackHeaderConfig.h#L42),
which [seems to be a legit way to hide the back
title](https://github.com/software-mansion/react-native-screens/blob/d54a19a/ios/RNSScreenStackHeaderConfig.mm#L514-L539).

**Test plan**

1. Create a native stack navigator with the [`backTitleVisible` screen
option set to
`false`](https://reactnavigation.org/docs/7.x/native-stack-navigator#headerbacktitlevisible).
2. See if the back title is hidden on iOS when navigating to another
screen.

| Expected | Not Expected |
| --- | --- |
|
![](https://github.com/react-navigation/react-navigation/assets/3784687/73a52ec8-df29-43ec-94e1-c515af93e0b7)
|
![](https://github.com/react-navigation/react-navigation/assets/3784687/89cd88bb-2e51-40e1-b4c8-7922ed6cda41)
|

* [x] May need to test if assigning the new property will break older
versions of `react-native-screens`.
* Tested with `react-native-screens` `3.20.0` and `3.21.0`, app does not
crash and the back title remains hidden-able.

packages/native-stack/src/views/HeaderConfig.tsx

@@ -181,7 +181,12 @@ export function HeaderConfig({
     <ScreenStackHeaderConfig
       backButtonInCustomView={backButtonInCustomView}
       backgroundColor={headerBackgroundColor}
-      backTitle={headerBackTitleVisible ? headerBackTitle : ' '}
+      backTitle={
+        headerBackTitleVisible
+          ? headerBackTitle
+          : ' ' /* For backward compatibility with react-native-screens versions <3.21.0, where `backTitleVisible` is not available */
+      }
+      backTitleVisible={headerBackTitleVisible}
       backTitleFontFamily={backTitleFontFamily}
       backTitleFontSize={backTitleFontSize}
       blurEffect={headerBlurEffect}