AST-1325 Silence CMake output for ska-sdp-func configuration

Previously ska-sdp-func has been added as dependency. If this library is not found the source code will be downloaded and build through FetchContent. The CMake configure stage of this library is quite verbose, obfuscating EveryBeams output.

This MR raises the log level to ERROR when ska-sdp-func is configured. Since this is not well supported for CMake versions lower than 3.17, a custom message() function is defined for these versions only, adding very basic log level support.

Testing the CMake<3.17 version was not straightforward. There is no spack package for cmake<3.17 available on das 6. The CI has an Ubuntu 20.04 build which comes with CMake 3.16. However, the Dockerfile uses pip install to get cmake, instead of apt-get. Pip installs a more recent version of CMake.

For testing the CI was modified to use apt-get to install cmake, 2fe77404 Also BASE_IMAGE_2004 was changed to force a rebuild 4da17912

These commits have been removed from the final MR, but a successful pipeline run with these commits can be found here https://git.astron.nl/RD/EveryBeam/-/pipelines/63274. The build-everybeam-2004 job there runs with cmake 3.16 https://git.astron.nl/RD/EveryBeam/-/jobs/628333