Upgrade GCC requirement to 10.2.0

https://bugs.webkit.org/show_bug.cgi?id=256078
rdar://108647034

Reviewed by Michael Catanzaro.

Based on the roadmap[1], we upgrade GCC requirement to 10.2.0.
This further unlocks many C++20 features, like constinit.

[1]: https://trac.webkit.org/wiki/WebKitGTK/GCCRequirement

* Source/cmake/WebKitCommon.cmake:

Canonical link: https://commits.webkit.org/263510@main

Source/cmake/WebKitCommon.cmake

@@ -67,8 +67,8 @@ if (NOT HAS_RUN_WEBKIT_COMMON)
     endif ()
 
     if (${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
-        if (${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS "9.3.0")
-            message(FATAL_ERROR "GCC 9.3 or newer is required to build WebKit. Use a newer GCC version or Clang.")
+        if (${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS "10.2.0")
+            message(FATAL_ERROR "GCC 10.2 or newer is required to build WebKit. Use a newer GCC version or Clang.")
         endif ()
     endif ()