Use the official pnggroup repos (#3099)

cmake/Modules/LocalZlibpng.cmake

diff --git a/cmake/Modules/LocalZlibpng.cmake b/cmake/Modules/LocalZlibpng.cmake
index 7101c0f..f87e6fd 100644
--- a/cmake/Modules/LocalZlibpng.cmake
+++ b/cmake/Modules/LocalZlibpng.cmake
@@ -52,7 +52,7 @@
 FetchContent_Declare(
     libpng
     EXCLUDE_FROM_ALL
-    GIT_REPOSITORY "https://github.com/glennrp/libpng.git"
+    GIT_REPOSITORY "https://github.com/pnggroup/libpng.git"
     GIT_TAG "${AVIF_LIBPNG_GIT_TAG}"
     GIT_SHALLOW ON
     UPDATE_COMMAND ""

ext/zlibpng.cmd

diff --git a/ext/zlibpng.cmd b/ext/zlibpng.cmd
index b0137b5..aafe954 100755
--- a/ext/zlibpng.cmd
+++ b/ext/zlibpng.cmd
@@ -8,4 +8,4 @@
 cd zlib
 git apply --ignore-whitespace ../zlib.patch
 cd ..
-git clone -b v1.6.55 --depth 1 https://github.com/glennrp/libpng.git
+git clone -b v1.6.55 --depth 1 https://github.com/pnggroup/libpng.git