cmake: bump minimum version to 3.16

This matches the current support matrix (from 2024-12-17) [1] and quiets
a warning from recent (3.31.5) versions of cmake:

CMake Deprecation Warning at CMakeLists.txt:12 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version
  of CMake.

Explicit setting of CMP0072 is also removed; it was added in 3.11.

[1]: https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

Bug: webp:397130631
Change-Id: Ic844dadf983a82674990edbddbfc54329df12eb7
Fixed: webp:397130631

CMakeLists.txt

@@ -9,11 +9,7 @@
 if(APPLE)
   cmake_minimum_required(VERSION 3.17)
 else()
-  cmake_minimum_required(VERSION 3.7)
-endif()
-
-if(POLICY CMP0072)
-  cmake_policy(SET CMP0072 NEW)
+  cmake_minimum_required(VERSION 3.16)
 endif()
 
 project(WebP C)