chore: Change constexpr to obj-c NSNumber (#3646)

Fixes
#3549

## Description

This PR replaces constexpr declaration with equivalent NSNumber
declaration for default font sizes. We weren't able to reproduce the
compilation error, but the change doesn't have a negative impact.

## Changes

Replaced the default font size declarations.

## Test plan

Have a header (regular, large) with some non-default font family (this
triggers the font size check). Set a breakpoint in
`RNSScreenStackHeaderConfig.mm:491` and `:519` and verify the constants
are correct.

## Checklist

- [ ] Ensured that CI passes

Author: kmichalikk

ios/RNSScreenStackHeaderConfig.mm

@@ -38,8 +38,8 @@
 namespace react = facebook::react;
 #endif // RCT_NEW_ARCH_ENABLED
 
-static constexpr auto DEFAULT_TITLE_FONT_SIZE = @17;
-static constexpr auto DEFAULT_TITLE_LARGE_FONT_SIZE = @34;
+static NSNumber *const DEFAULT_TITLE_FONT_SIZE = @17;
+static NSNumber *const DEFAULT_TITLE_LARGE_FONT_SIZE = @34;
 
 #if !defined(RCT_NEW_ARCH_ENABLED)
 // Some RN private method hacking below. Couldn't figure out better way to access image data