revert modules back to clang-20 to see if CI cleans-up

Author: JeffGarland

CMakeLists.txt

@@ -11,8 +11,8 @@ project(
 
 # gersemi: off
 
-# Modules opt in only on compilers that support g++-15 and clang-19+
-if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19)
+# Modules opt in only on compilers that support g++-15 and clang-20+
+if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 20)
     set(CMAKE_CXX_SCAN_FOR_MODULES 1)
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 15)
     set(CMAKE_CXX_SCAN_FOR_MODULES 1)