Update json files for 1.2

This was previously done for CMake builds in commit 56f1644, but not
GN. Without this change the loader will output this warning:
WARNING: loader_add_to_layer_list: Explicit layer VK_LAYER_KHRONOS_validation is using an old API version 1.1 versus application requested 1.2

Change-Id: I30017cef51a70b6907ad82df50712eebf81c811b
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/Vulkan-ValidationLayers/+/542442
Reviewed-by: Craig Stout <cstout@google.com>
diff --git a/build-gn/generate_vulkan_layers_json.py b/build-gn/generate_vulkan_layers_json.py
index 5a0ced5..33f6b08 100755
--- a/build-gn/generate_vulkan_layers_json.py
+++ b/build-gn/generate_vulkan_layers_json.py
@@ -129,7 +129,7 @@
             for line in infile:
                 line = line.replace('@RELATIVE_LAYER_BINARY@',
                                     relative_path_prefix + layer_lib_name)
-                line = line.replace('@VK_VERSION@', '1.1.' + vk_version)
+                line = line.replace('@VK_VERSION@', '1.2.' + vk_version)
                 json_out_file.write(line)
 
 if __name__ == '__main__':