Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

docs/README-migration.md

@@ -4,6 +4,8 @@ This guide provides useful information for migrating applications from SDL 2.0 t
 
 We have provided a handy Python script to automate some of this work for you [link to script], and details on the changes are organized by SDL 2.0 header below.
 
+SDL headers should now be included as `#include <SDL3/SDL.h>`. Typically that's the only header you'll need in your application unless you are using OpenGL or Vulkan functionality.
+
 
 ## SDL_platform.h