[build] Switch lz4 submodule to upstream lz4/lz4

.github/skills/update-tpn/SKILL.md

@@ -56,7 +56,7 @@ Read `.gitmodules` for all submodules. Current submodules and their license file
 | Submodule | URL | License File |
 |-----------|-----|-------------|
 | Java.Interop | https://github.com/dotnet/java-interop | `external/Java.Interop/LICENSE` |
-| lz4 | https://github.com/dotnet/lz4 (fork of https://github.com/lz4/lz4) | `external/lz4/lib/LICENSE` |
+| lz4 | https://github.com/lz4/lz4 | `external/lz4/lib/LICENSE` |
 | xxHash | https://github.com/Cyan4973/xxHash | `external/xxHash/LICENSE` |
 | constexpr-xxh3 | https://github.com/chys87/constexpr-xxh3 | `external/constexpr-xxh3/LICENSE` |
 | robin-map | https://github.com/xamarin/robin-map (fork of https://github.com/Tessil/robin-map) | `external/robin-map/LICENSE` |

.gitmodules

@@ -20,8 +20,8 @@
     branch = v1.8-stable
 [submodule "external/lz4"]
     path = external/lz4
-    url = https://github.com/dotnet/lz4
-    branch = release
+    url = https://github.com/lz4/lz4
+    branch = dev
 [submodule "external/robin-map"]
     path = external/robin-map
     url = https://github.com/xamarin/robin-map