Drop CMake 3 support and standardize on CMake 4

[brondani]

Describe The Problem To Be Solved

We currently maintain compatibility and test coverage for both CMake 3 and CMake 4. This creates additional maintenance overhead across our generated CMakeLists.txt, CI infrastructure, and test matrix.
Supporting older CMake versions has several drawbacks:

• Increased CI execution time and infrastructure cost due to maintaining multiple version combinations.
• Reduced ability to adopt modern CMake features and best practices.
• More effort required to validate changes across multiple major CMake generations.
• Continued exposure to deprecated behaviors and compatibility paths that are being removed upstream.

CMake 4 removes support for a number of legacy compatibility modes and represents the direction of the CMake ecosystem going forward. Continuing to support CMake 3 provides diminishing value while increasing maintenance burden.

Suggest A Solution

Remove support for CMake 3 and standardize development, testing, and generated build files on CMake 4.
This includes:

• Removing CMake 3 jobs from the CI matrix.
• Updating the minimum supported CMake version to a CMake 4 release.
• Updating documentation and examples to reference CMake 4.

Benefits

• Smaller and simpler maintenance surface.
• Reduced CI runtime and infrastructure requirements.
• Faster adoption of modern CMake capabilities.
• Improved consistency across supported environments.
• Alignment with the long-term direction of the CMake project.
• Reduced risk associated with legacy compatibility paths.

Optional Transition Strategy

If an immediate removal of CMake 3 support is considered too disruptive, we could introduce a deprecation phase before enforcing a CMake 4 minimum.
During this period we would continue generating projects with a CMake 3 minimum requirement while recommending CMake 4 and emitting a warning when an older version is used:

cmake_minimum_required(VERSION 3.27)
set(RECOMMENDED_CMAKE_VERSION 4.2.1)
if(CMAKE_VERSION VERSION_LESS ${RECOMMENDED_CMAKE_VERSION})
  message(WARNING
    "CMake ${RECOMMENDED_CMAKE_VERSION} or newer is recommended. "
    "You are running version ${CMAKE_VERSION}")
endif()

This approach allows users time to upgrade while giving us visibility into the upcoming support change. After the deprecation period, the minimum required version can be raised to CMake 4 and the remaining compatibility code removed.