[BUG] Build failed in old MacOS (10.13 High Sierra) since fastfetch 2.21.2

[niu541412]

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General description of bug:

• What happened:
  The following is the build log:

...
[ 77%] Building C object CMakeFiles/libfastfetch.dir/src/detection/displayserver/displayserver_apple.c.o
/Users/username/Downloads/fastfetch/src/detection/displayserver/displayserver_apple.c:92:21: error: use of undeclared identifier 'CoreDisplay_Display_IsHDRModeEnabled'
                if (CoreDisplay_Display_IsHDRModeEnabled)
/Users/username/Downloads/fastfetch/src/detection/displayserver/displayserver_apple.c:94:43: error: implicit declaration of function 'CoreDisplay_Display_IsHDRModeEnabled' is
      invalid in C99 [-Werror,-Wimplicit-function-declaration]
                    display->hdrEnabled = CoreDisplay_Display_IsHDRModeEnabled(screen);
...

This error is intruduced from the commit ea18d93, because CoreDisplay_Display_IsHDRModeEnabled
is called in all MacOS version, but it's only provided since MacOS 10.15. It can be fixed with macro condition statement or Null pointer checking of CoreDisplay_Display_IsHDRModeEnabled.