[Copilot]

Address review comments on the C++20 migration: add CMAKE_CXX_STANDARD_REQUIRED ON so configure fails early on incompatible toolchains, and fix style inconsistencies in the Homme block.

Changes

• components/CMakeLists.txt — Bump to C++20 and add CMAKE_CXX_STANDARD_REQUIRED ON
• components/eamxx/CMakeLists.txt — Replace the if (NOT CMAKE_CXX_STANDARD) guard with an unconditional set + required, matching components/CMakeLists.txt
• components/homme/CMakeLists.txt — Add C++20 default inside the standalone IF block, using the file's existing uppercase commands and proper indentation

# components/CMakeLists.txt and components/eamxx/CMakeLists.txt
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# components/homme/CMakeLists.txt (inside IF (NOT DEFINED PROJECT_NAME))
  IF (NOT CMAKE_CXX_STANDARD)
    SET (CMAKE_CXX_STANDARD 20)
  ENDIF ()