Avoid warning of deprecated cmake 3.5 by removing mention of cmake 3.3

[wdeconinck]

This avoids following warning for each project configured with cmake 3.27 onwards:

CMake Deprecation Warning at /home/runner/work/_temp/install/ecbuild/share/ecbuild/cmake/ecbuild_project.cmake:44 (cmake_minimum_required):
    Compatibility with CMake < 3.5 will be removed from a future version of
    CMake.
  
    Update the VERSION argument <min> value or use a ...<max> suffix to tell
    CMake that the project does not need compatibility with older versions.
  Call Stack (most recent call first):
    CMakeLists.txt:24 (project)